If you listen ___________ to this piece of music, you can hear a flute in the background.

If you listen more closely to this song, you can hear the flute in the background.

Consider the answers:

A. careful → A is wrong because after the verb listen is an adverb to modify it, so the adjective “careful” cannot come after “listen” here, it must be “careful”-> “carefully”.

B. more carefully → Yes

C. more careful → There is no “than” so this is not a superlative and the blank needs an adverb

D. most careful → Missing the article “the” and the blank need an adverb

The answer is NO