(A) Dumbness - Lacking intelligence or good judgment
(B) Changeableness - the ability to undergo change of any sort
(C) Hesitation - the action of pausing or hesitating before saying or doing something.
(D) Reserved - slow to reveal emotion or opinions.
The given word Taciturnity means (of a person) to be reserved or uncommunicative in speech; saying little. From the definition of the word, option (D) is the correct answer. Option (C) is only situational and option (A) and option (B) are unrelated. Hence the answer is option (D).