Find out whether there is any grammatical error in below sentence. I fail to understand / why he replied in negative / when the proposal was in his favour.

The correct answer is D. No error.

The sentence is grammatically correct. The word “why” is used to introduce a clause that asks for a reason. The word “in negative” is used to describe the way in which someone replied. The word “when” is used to introduce a clause that describes the time at which something happened.

The sentence can be

paraphrased as follows: “I do not understand why he replied in the negative when the proposal was in his favour.”