Known for his ____________ nature, the old librarian preferred to communicate through notes rather than engage in lengthy conversations.
Correct: D
The context describes someone who prefers notes over lengthy conversations, indicating a disinclination to talk much. 'Taciturn' means reserved or uncommunicative in speech; saying little. This is the best fit. 'Loquacious', 'garrulous', and 'voluble' all mean talkative or speaking a great deal, which are antonyms.