In English vocabulary, what do we call a person who pretends to be something that he or she is not?

Introduction / Context:
This is a classic one-word substitution question from English vocabulary. It asks you to identify the correct term for a person who pretends to be what he or she is not. Such a person may fake qualifications, status, or identity. Recognising this word is useful in both everyday language and exam passages, where characters are sometimes described using such labels.

Given Data / Assumptions:

• The definition in the stem is a person who pretends to be what he is not.
• Option A posthumous relates to something that happens after a person’s death.
• Option B glutton means someone who eats or drinks too much.
• Option C impostor refers to a person who deceives others by pretending to be someone else.
• Option D rogue is a dishonest or unprincipled person, but not necessarily one who assumes a false identity.

Concept / Approach:
The key idea in the question is deliberate pretence about true identity, character, or status. An impostor is exactly such a person: someone who poses as another person or falsely claims to have certain qualifications. A rogue may cheat or misbehave, but the focus there is general dishonesty, not the assumption of a completely false identity. Posthumous and glutton obviously belong to different semantic fields (life events and eating habits), so they can be rejected quickly.

Step-by-Step Solution:
Step 1: Underline the phrase pretends to be what he is not in the question.Step 2: Recall the standard dictionary definition of impostor: a person who pretends to be someone else in order to deceive others.Step 3: Examine posthumous and note that it describes events after death, such as a posthumous award; it does not describe a person who is pretending.Step 4: Examine glutton and remember it refers to greed in eating or drinking, again unrelated to identity or pretence.Step 5: Examine rogue and see that although it can mean a dishonest person, the word does not specifically convey the idea of pretending to be someone else. Thus impostor is the most accurate match.

Verification / Alternative check:
We can check by placing each option into a sample sentence. Consider The man who posed as a doctor without a medical degree was an ____. The natural completion is impostor, because he pretended to be a doctor. Calling him a glutton or posthumous would be absurd, and calling him a rogue would be too vague. News reports and novels commonly use the phrase an impostor when someone fakes their identity, which confirms the correctness of this choice.

Why Other Options Are Wrong:
Posthumous is an adjective, not a noun for a person, and it refers to events after someone has died. Glutton is about overeating, not about deception in identity. Rogue is a general term for a dishonest or mischievous person, but the exam is clearly asking for the specific word meaning someone who pretends to be what he or she is not, which is covered precisely by impostor.

Common Pitfalls:
Students sometimes pick rogue because they associate dishonesty with pretending. However, one-word substitution questions usually aim for very specific matches. Another pitfall is confusion between similar spelling forms like impostor and imposter; both appear in modern English, but impostor is the traditional dictionary spelling. In exams, knowing the standard form is helpful.

Final Answer:
The correct one-word term for a person who pretends to be what he is not is Impostor, so option C is correct.