The correct answer is: A. Reserve.
Reserve is a synonym of taciturnity because it means to be quiet and reserved in speech or manner. Someone who is taciturn is not very talkative and may be difficult to get to know. They may be shy or introverted, or they may simply prefer to keep their thoughts to themselves.
Hesitation is not a synonym of taciturnity because it means to be slow or reluctant to act or speak. Someone who is hesitant may be unsure of themselves or what to do, or they may be afraid of making a mistake.
Changeableness is not a synonym of taciturnity because it means to be likely to change one’s mind or behavior. Someone who is changeable may be difficult to rely on or trust, as they may not always do what they say they will do.
Dumbness is not a synonym of taciturnity because it means to be stupid or lacking in intelligence. Someone who is dumb may have difficulty understanding or speaking, or they may simply not be very bright.
In conclusion, the correct answer is A. Reserve.