Final answer:
The meaning of 'taciturn' is revealed through a contrast context clue, established by contrasting the descriptions of a talkative boy and a reserved, taciturn girl.
Step-by-step explanation:
The context clue for determining the meaning of the underlined word taciturn in the set of sentences provided is a contrast context clue. The boy is described as a lively conversationalist, which denotes someone who is talkative and enjoys engaging in conversations. Conversely, the girl is described as reserved and taciturn, establishing a contrast that helps to deduce that taciturn means someone who is quiet and speaks infrequently. This contrast in the character description between the boy and the girl reveals the meaning of taciturn without directly stating it.