In each of the following questions, find out which part of the sentence has an error. The sentence is divided into three parts labelled A, B and C, and part D indicates that the sentence is correct. Identify the part that contains a grammatical or usage error in the sentence: The man who they thought to be a gentleman turned out to be a rougue.

Introduction / Context:
This question tests your ability to spot errors in a complex sentence where both grammar and spelling may cause confusion. The sentence describes a man who appeared to be a gentleman but was later revealed to be something else. You must inspect each part carefully to find the portion that contains a clear mistake in standard written English or decide that the sentence is correct.

Given Data / Assumptions:
- Complete sentence: The man who they thought to be a gentleman turned out to be a rougue. - Part A: The man who they thought to be. - Part B: a gentleman turned out. - Part C: to be a rougue. - Part D: The sentence is grammatically correct; no error.

Concept / Approach:
There are two aspects to check here. The first is the relative clause structure with who they thought to be, and the second is the spelling of key words. While advanced grammar discussions may consider whether who or whom is preferable, examination questions at this level often highlight more obvious errors such as incorrect spellings. The word rougue in part C is not a correct English spelling. The intended word is rogue, meaning a dishonest or unprincipled man. Therefore, the clearest and most definite error lies in part C.

Step-by-Step Solution:
Step 1: Examine part A, The man who they thought to be. This introduces the relative clause and, although some grammarians might prefer whom instead of who, the structure is widely understood and commonly used in modern English. Step 2: Examine part B, a gentleman turned out. This segment correctly indicates that the man was believed to be a gentleman and forms part of the predicate turned out. Step 3: Examine part C, to be a rougue. The context indicates that he actually turned out to be a dishonest person. The standard English spelling for this word is rogue, not rougue. Step 4: Since part C clearly contains a misspelt word, it is the part that must be marked as having an error.

Verification / Alternative check:
Rewrite the sentence with correct spelling: The man who they thought to be a gentleman turned out to be a rogue. Now the sentence reads smoothly and the meaning is clear. No other segment stands out as a definite error at this level of testing. This confirms that the key mistake is confined to the word rougue in part C, which must be corrected to rogue. Therefore, selecting part C as the error location is justified.

Why Other Options Are Wrong:
Part A is acceptable in common usage, and the relative clause structure is understandable and functional. Part B correctly expresses the idea that he was believed to be a gentleman and contains no structural error. Part D, claiming no error, is wrong because the misspelling in part C makes the sentence incorrect.

Common Pitfalls:
Students sometimes focus exclusively on advanced points like who versus whom and overlook clear spelling mistakes. While precision is useful, in examination conditions you should first identify obvious errors such as incorrect spellings, wrong verb forms, or missing prepositions. Another pitfall is reading too quickly and mentally correcting the spelling without noticing it is wrong on the page. Reading every key content word carefully and comparing it with known spellings helps avoid such oversights.

Final Answer:
The error lies in the part to be a rougue, where rougue must be corrected to rogue.