The correct option is A Reserved
A reserved person is slow to reveal emotion or opinions, and thereby, talks usually less. For example, "he is a reserved, almost taciturn man". Hence, Option A is correct. Among the rest of the options, mute means, phlegmatic means (of a person) having an unemotional and stolidly calm disposition, stoic means person who can endure pain or hardship without showing their feelings or complaining. These options do not express the intended meaning, hence incorrect.