AZ-900 Describe cloud concepts Practice Question
A company is migrating a custom-built inventory management application to the cloud. The application runs on a specific version of Windows Server and requires custom registry settings that are not supported in a platform as a service (PaaS) offering. The company wants to avoid the overhead of managing physical servers but still needs full control over the operating system and application dependencies. Which cloud service model should the company use?
⚠ Common exam trap
It's easy for candidates to choose PaaS because it reduces management overhead, but they overlook the explicit requirement for custom registry settings and full OS control, which only IaaS can provide.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Infrastructure as a Service (IaaS)
The company needs full control over the operating system and custom registry settings, which are not supported in PaaS. Infrastructure as a Service (IaaS) provides virtualized computing resources (e.g., virtual machines) where the customer manages the OS, applications, and dependencies, while the cloud provider handles the physical hardware. This model allows the company to run a specific version of Windows Server with custom registry configurations without managing physical servers.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Infrastructure as a Service (IaaS)
Why this is correct
IaaS is correct because it provides virtual machines with full administrative control over the guest operating system. You can install custom software, modify Windows Registry settings (e.g., via `regedit`), and choose a specific Windows Server version or build, ensuring compatibility with your legacy inventory app. While you must manage the OS yourself, IaaS is the only option that exposes the OS layer directly to you.
- ✗
Platform as a Service (PaaS)
Why it's wrong here
PaaS is incorrect because it abstracts and manages the underlying operating system and runtime infrastructure. With services like Azure App Service, you deploy only your code and configure application settings, but the host OS is fixed, patched, and isolated from you; there is no way to access the registry or select a custom Windows Server version. This lack of OS-level control explicitly rules out PaaS for this migration.
When this WOULD be correct
A company wants to deploy a web application using a built-in runtime (e.g., .NET) without managing OS patches or scaling infrastructure. They need automatic scaling and high availability, and the application does not require custom OS-level configurations.
- ✗
Software as a Service (SaaS)
Why it's wrong here
SaaS is incorrect because it delivers a fully managed, ready-to-use application over the internet (e.g., Office 365, Salesforce). The vendor owns and operates the entire stack—operating system, runtime, and application—so you have zero ability to modify OS-level configuration, registry, or the Windows Server version. Your control is limited to application-level settings within the provided user interface.
When this WOULD be correct
A company wants to use a cloud-based email service without managing servers or software. They need a subscription-based model where the provider handles all maintenance, updates, and security. SaaS would be the correct choice.
- ✗
Function as a Service (FaaS)
Why it's wrong here
FaaS is incorrect because it takes serverless abstraction to the extreme: your code runs in a stateless, event-triggered environment (e.g., Azure Functions) where the underlying OS is invisible and completely managed by the platform. You cannot modify registry settings, install OS components, or even choose a Windows Server version; the runtime and scaling are automatically governed by the service. Such constraints make FaaS unsuitable for an application that requires persistent OS-level customization.
When this WOULD be correct
A company wants to run event-driven code (e.g., processing uploaded images) without managing servers, paying only for execution time, and the code can be written in a supported language like C# or Python with no OS-level dependencies.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-900 exam frequently reuses these exact scenarios with slightly different constraints.
✓Infrastructure as a Service (IaaS)Correct answer▾
Why this is correct
IaaS is correct because it provides virtual machines with full administrative control over the guest operating system. You can install custom software, modify Windows Registry settings (e.g., via `regedit`), and choose a specific Windows Server version or build, ensuring compatibility with your legacy inventory app. While you must manage the OS yourself, IaaS is the only option that exposes the OS layer directly to you.
✗Platform as a Service (PaaS)Wrong answer — click to see why▾
Why this is wrong here
PaaS does not provide control over the underlying operating system or custom registry settings; it abstracts the OS, making it unsuitable for applications requiring specific Windows Server configurations.
★ When this WOULD be the correct answer
A company wants to deploy a web application using a built-in runtime (e.g., .NET) without managing OS patches or scaling infrastructure. They need automatic scaling and high availability, and the application does not require custom OS-level configurations.
Why candidates choose this
Candidates may confuse PaaS with IaaS, thinking PaaS still offers some OS control, or they may overlook the specific requirement for custom registry settings that PaaS cannot accommodate.
✗Software as a Service (SaaS)Wrong answer — click to see why▾
Why this is wrong here
SaaS provides ready-to-use software accessed via the internet, with no control over the underlying OS or custom registry settings. The company needs full control over the OS and application dependencies, which SaaS cannot offer.
★ When this WOULD be the correct answer
A company wants to use a cloud-based email service without managing servers or software. They need a subscription-based model where the provider handles all maintenance, updates, and security. SaaS would be the correct choice.
Why candidates choose this
Candidates may think SaaS is suitable because it is a common cloud service model, but they overlook the requirement for full control over the OS and custom configurations, which SaaS does not provide.
✗Function as a Service (FaaS)Wrong answer — click to see why▾
Why this is wrong here
FaaS (e.g., Azure Functions) is a serverless compute service that abstracts the underlying OS and runtime, providing no control over Windows Server version or custom registry settings, which are required by the application.
★ When this WOULD be the correct answer
A company wants to run event-driven code (e.g., processing uploaded images) without managing servers, paying only for execution time, and the code can be written in a supported language like C# or Python with no OS-level dependencies.
Why candidates choose this
Candidates may confuse FaaS with IaaS because both involve 'functions' or 'infrastructure' in the name, or they mistakenly think FaaS provides OS-level control since it runs code in a cloud environment.
Analysis generated from the official AZ-900blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
What is Cloud Computing?
Key term
IaaS
IaaS stands for Infrastructure as a Service, which means renting virtual computing resources like servers, storage, and networking from a cloud provider instead of buying and managing physical hardware yourself.
Key term
PaaS
Platform as a Service (PaaS) is a cloud computing model that provides a managed platform for developers to build, run, and manage applications without dealing with the underlying infrastructure.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-900 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the AZ-900 exam.