- A
Using a JSON key file downloaded for the service account.
Service account key files can be used for authentication.
- B
Using a user account's OAuth2 tokens obtained via a web browser.
Why wrong: User account tokens are for interactive flows, not for server-side applications.
- C
Using an API key generated from the Cloud Console.
Why wrong: API keys are for identifying projects, not for service account authentication.
- D
Using the Compute Engine metadata server to obtain an access token for a custom service account.
You can call the metadata server to get a token for any service account attached to the instance.
- E
Using the default service account's automatically provided credentials.
The default service account's credentials are available via the metadata server.
Google ACE Practice Question: Ensuring successful operation of a cloud solution
This ACE practice question tests your understanding of ensuring successful operation of a cloud solution. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which THREE options are valid methods to authenticate a service account when making calls to Google Cloud APIs from a Compute Engine instance?
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
Using a JSON key file downloaded for the service account.
Option A is correct because a JSON key file downloaded for a service account contains the private key necessary to create a signed JWT assertion, which is exchanged for an OAuth 2.0 access token via the Google OAuth 2.0 token endpoint (https://oauth2.googleapis.com/token). This is a standard authentication method for service accounts outside of Google Cloud, but it is also valid from a Compute Engine instance, though less secure than using the metadata server.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Using a JSON key file downloaded for the service account.
Why this is correct
Service account key files can be used for authentication.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Using a user account's OAuth2 tokens obtained via a web browser.
Why it's wrong here
User account tokens are for interactive flows, not for server-side applications.
- ✗
Using an API key generated from the Cloud Console.
Why it's wrong here
API keys are for identifying projects, not for service account authentication.
- ✓
Using the Compute Engine metadata server to obtain an access token for a custom service account.
Why this is correct
You can call the metadata server to get a token for any service account attached to the instance.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Using the default service account's automatically provided credentials.
Why this is correct
The default service account's credentials are available via the metadata server.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between authentication (proving identity) and authorization (granting permissions), and the trap here is that candidates mistakenly think API keys (Option C) can authenticate a service account, when in fact API keys only identify the project and are not tied to a specific identity.
Detailed technical explanation
How to think about this question
Under the hood, the Compute Engine metadata server exposes a REST endpoint at http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/ that returns OAuth2 access tokens for any attached service account, including custom ones, without needing to manage keys. The default service account is automatically attached to the instance and its credentials are obtained via the same metadata server, using the 'default' alias. This mechanism leverages the instance's identity and avoids the security risk of storing private keys on disk.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this ACE question test?
Ensuring successful operation of a cloud solution — This question tests Ensuring successful operation of a cloud solution — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Using a JSON key file downloaded for the service account. — Option A is correct because a JSON key file downloaded for a service account contains the private key necessary to create a signed JWT assertion, which is exchanged for an OAuth 2.0 access token via the Google OAuth 2.0 token endpoint (https://oauth2.googleapis.com/token). This is a standard authentication method for service accounts outside of Google Cloud, but it is also valid from a Compute Engine instance, though less secure than using the metadata server.
What should I do if I get this ACE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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