MS-900 Describe cloud concepts Practice Question
A company runs a custom application on a cloud provider's infrastructure. The provider manages the physical servers, networking, and storage, but the company installs, configures, and patches the operating system and application. Which cloud service model is this?
⚠ Common exam trap
Candidates often confuse IaaS with PaaS because both involve deploying applications, but the key differentiator is OS-level control and patching responsibility—IaaS gives you full OS access, while PaaS abstracts it away.
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)
This scenario describes Infrastructure as a Service (IaaS) because the cloud provider manages the underlying physical infrastructure (servers, networking, storage), while the customer retains control over the operating system, middleware, and application. In IaaS, the customer is responsible for OS patching, configuration, and application management, which matches the given responsibilities.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Software as a Service (SaaS)
Why it's wrong here
SaaS delivers a fully managed, ready-to-use software application over the internet, such as Microsoft 365. The provider owns the entire stack—data center, network, servers, operating system, and application—so the customer only logs on and configures settings. Because the company here runs a custom application and presumably controls the OS, a multi-tenant or single-tenant SaaS offering would not give it the required flexibility or administrative access.
- ✗
Platform as a Service (PaaS)
Why it's wrong here
PaaS supplies an application-hosting platform, including the underlying servers, operating system, runtime, and middleware, which the provider patches and updates. Customers deploy their application code or containers but never manage the guest OS, so they cannot perform OS-level patching, configure the OS registry, or install custom system components. The scenario described is IaaS because the customer is responsible for those OS management tasks, not PaaS.
- ✓
Infrastructure as a Service (IaaS)
Why this is correct
IaaS provides virtualized compute, storage, and networking as raw infrastructure blocks, with the provider maintaining only the physical data center, hosts, hypervisor, and network fabric. The customer provisions virtual machines, installs and patches the guest operating system, configures storage and firewall rules, and runs the custom application on top. In the scenario, the customer has control over the OS and app stack, which is exactly the IaaS responsibility model.
- ✗
Function as a Service (FaaS)
Why it's wrong here
FaaS runs individual, event-driven code functions in a stateless, serverless execution environment; all host, OS, and runtime infrastructure is abstracted and auto-managed by the provider. It is billed per execution and scales every function independently, making it a poor fit for a long-running, stateful custom application that needs a specific OS environment. Since the customer here manages the OS and operates a custom application as a persistent workload, FaaS does not match.
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