The answer is using the gcloud CLI default application credentials, along with the environment variable GOOGLE_APPLICATION_CREDENTIALS and the metadata server on Google Cloud resources. These three methods form the core of Application Default Credentials (ADC), which is the unified authentication strategy for Google Cloud client libraries. ADC automatically searches for credentials in this order: first the environment variable, then the gcloud CLI’s default credentials, and finally the metadata server when running on Compute Engine, Cloud Run, or other Google Cloud services. On the Google Professional Cloud Developer exam, this question tests your understanding of how service account authentication methods work in code, not just in the console. A common trap is confusing API keys or OAuth 2.0 client IDs with service account authentication—API keys identify projects, not service accounts, and OAuth client IDs are for end-user login. Remember the ADC order as “Env, gcloud, Metadata” to recall the three valid paths.
PCD Integrating Google Cloud services Practice Question
This PCD practice question tests your understanding of integrating google cloud services. 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 are valid ways to authenticate a service account when using the Google Cloud client libraries? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Attaching a service account to a Compute Engine instance and letting the metadata server provide credentials.
Options A, B, and C are correct. These are the standard methods for Application Default Credentials (ADC). Option D is wrong because API keys are not used for service account authentication. Option E is wrong because OAuth 2.0 client IDs are for user authentication.
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.
✓
Attaching a service account to a Compute Engine instance and letting the metadata server provide credentials.
Why this is correct
This is the automatic method on Compute Engine.
Related concept
Read the scenario before looking for a memorised answer.
✓
Setting the GOOGLE_APPLICATION_CREDENTIALS environment variable to point to a service account key file.
Why this is correct
This is a standard method for ADC.
Related concept
Read the scenario before looking for a memorised answer.
✗
Using OAuth 2.0 client IDs for web applications.
Why it's wrong here
OAuth client IDs are for user authentication, not service accounts.
✓
Using the gcloud CLI default application credentials.
Why this is correct
ADC picks up credentials from gcloud when running locally.
Related concept
Read the scenario before looking for a memorised answer.
✗
Using an API key in the client library initialization.
Why it's wrong here
API keys are not service account credentials.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Integrating Google Cloud services — This question tests Integrating Google Cloud services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Attaching a service account to a Compute Engine instance and letting the metadata server provide credentials. — Options A, B, and C are correct. These are the standard methods for Application Default Credentials (ADC). Option D is wrong because API keys are not used for service account authentication. Option E is wrong because OAuth 2.0 client IDs are for user authentication.
What should I do if I get this PCD question wrong?
Identify which PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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