The correct answer is **conscientious** (Option 2). **Comprehensive Derivation:** The correct answer is "conscientious" Explanation & Key Takeaways: • In the above question, we have to fill in the correct form of the adjective to fill in the given blank. • The adjective "conscientious" is the most appropriate choice for this sentence because it means "having a strong sense of duty and responsibility." This is in line with the idea expressed in the sentence that the man has done his duty and is therefore a person who takes his responsibilities seriously. • The other options are incorrect because they do not fit the context of the sentence as well as "conscientious": • "upright" is incorrect because it means "having a strong moral character and behaving in an honest and fair way," which is similar to the idea expressed in the sentence, but "conscientious" better captures the idea of a strong sense of duty and responsibility. • "conscious" is incorrect because it means "aware of one's surroundings or situation," which is not relevant to the context of the sentence. • "truthful" is incorrect because it means "not lying or deceiving others," which is not the same as having a strong sense of duty and responsibility as expressed in the sentence. • Thus, as per the above explanation the correct answer is Option 2. Correct Sentence: He must have done his duty because he is a conscientious man. **Option Elimination:** • **Option 1 (upright)**: Incorrect distractor. • **Option 2 (conscientious)**: Correct Answer; accurate according to canonical reference. • **Option 3 (conscious)**: Incorrect distractor. • **Option 4 (truthful)**: Incorrect distractor. **Exam Trap:** Be careful of misleading distractors and subtle terminology differences. Verify core properties before selecting your answer. **Memory Booster:** Core mnemonic rule: conscientious corresponds to Option (2).