This KCNA practice question tests your understanding of container orchestration. 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.
A container image is built from a Dockerfile with multiple layers. Which statement about container image layers is TRUE?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Layers are read-only and can be reused across different images
Option C is correct because container image layers are read-only and are stored in a content-addressable storage (e.g., overlayfs, aufs). These layers can be reused across different images when they share the same content hash, which is a fundamental efficiency of Docker's union filesystem. This layer sharing reduces disk usage and speeds up image pulls.
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.
✗
Each layer is created by a RUN instruction and can be modified after the image is built
Why it's wrong here
Image layers are immutable; they cannot be modified after the image is built.
✗
Each layer is unique to the image and cannot be shared with other images
Why it's wrong here
Layers are often shared between images, especially base layers, to reduce storage.
✓
Layers are read-only and can be reused across different images
Why this is correct
Image layers are read-only and are shared across images that use the same base or intermediate layers, improving efficiency.
Related concept
Read the scenario before looking for a memorised answer.
✗
All layers in a container image are writable at runtime
Why it's wrong here
Only the topmost container layer is writable; image layers are read-only.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CNCF often tests the misconception that all layers are writable at runtime, but in reality only the container's writable layer is mutable, while the underlying image layers remain read-only.
Detailed technical explanation
How to think about this question
Under the hood, Docker uses a union filesystem (e.g., overlay2) where each layer is a directory containing filesystem changes. When pulling an image, only layers not already present locally are downloaded, thanks to content-addressable storage. In a real-world CI/CD pipeline, this layer caching dramatically reduces build times when only the top layers change.
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 practitioner preparing for the KCNA exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this KCNA question in full detail.
Container Orchestration — This question tests Container Orchestration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Layers are read-only and can be reused across different images — Option C is correct because container image layers are read-only and are stored in a content-addressable storage (e.g., overlayfs, aufs). These layers can be reused across different images when they share the same content hash, which is a fundamental efficiency of Docker's union filesystem. This layer sharing reduces disk usage and speeds up image pulls.
What should I do if I get this KCNA 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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Question Discussion
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