Bitter :

The answer to the question “Bitter : A. Blunt B. Bold C. Timid D. Sweet” is A. Blunt.

Bitter is an adjective that describes a taste that is unpleasantly sharp or sour. Blunt is an adjective that describes someone who is direct and honest, sometimes to the point of being rude. Bold is an adjective that describes someone who is brave and courageous. Timid is an adjective that describes someone who is shy and nervous. Sweet is an adjective that describes a taste that is pleasant and sugary.

Of the four options, only blunt is similar to bitter in that it is both unpleasant and direct. The other options are not similar to bitter in either of these ways.