Choose the one-word substitute for the expression "Deviation from the right course".

Introduction / Context:
This question asks you to find a one-word substitute for a phrase describing a departure from what is right, normal, or expected. Such vocabulary is often used in moral, scientific, and social discussions where behaviour or results differ from the usual pattern.

Given Data / Assumptions:

• The phrase to be replaced is "Deviation from the right course".
• The options are "Imagination", "Amalgamation", "Illumination", and "Aberration".
• Standard English dictionary meanings apply.
• Only one option must convey the idea of straying away from correctness or normality.

Concept / Approach:
The phrase suggests moving away from a correct or normal path, either literally or figuratively. The word "aberration" is commonly used to describe a departure from what is normal, usual, or expected, often in an undesirable way. Therefore we must check whether "aberration" matches the phrase more closely than the other options, which refer to completely different concepts.

Step-by-Step Solution:
Step 1: Examine "Aberration". It means a deviation from what is normal or correct, especially something that is unwelcome or unhealthy.Step 2: Compare with the phrase "Deviation from the right course". Both emphasise moving away from a proper or standard path.Step 3: Examine "Imagination". This refers to the mental ability to form new ideas or images; it is not about deviation from correctness.Step 4: Examine "Amalgamation". This means the process of combining or merging things, often used for organisations or metals, which is unrelated to deviation.Step 5: Examine "Illumination". This means lighting or enlightenment, either physical or mental, not departure from a right course.Step 6: Since "Aberration" alone matches the idea of deviation from a right or normal path, it is the correct answer.

Verification / Alternative check:
Example sentences confirm this meaning: "The scientist considered the unusual result an aberration" and "His rude behaviour was an aberration from his usual politeness". In both cases, "aberration" signifies a departure from the expected or right pattern. This corresponds directly with "Deviation from the right course".

Why Other Options Are Wrong:
Option A, "Imagination", is associated with creativity and mental imagery, not deviation from correctness.
Option B, "Amalgamation", is about mixing or uniting things, the opposite of deviation or separation.
Option C, "Illumination", is about light or understanding and has no natural connection to going off the right course.

Common Pitfalls:
Some candidates may be tempted by options that simply sound impressive or sophisticated. Others may confuse "aberration" with words like "abbreviation" or "abomination" purely due to sound. It is crucial to know not only the sound but the precise meaning of these advanced vocabulary items to perform well in one-word substitution questions.

Final Answer:
The correct one-word substitute is "Aberration", so option D is correct.