Explanation

The sentence sets up a series of contrasts between the two described people, so the blank needs to mean the opposite of “garrulous”, which means “overly talkative”. Choices D
and F ​are correct: both “laconic” and “taciturn” mean “shy,” “using few words” or “reluctant to speak”.

Thank you for your response. Do you have any trick/strategy when one is not familiar with any of the words in an answer choice? this could be roots etc..

While in many ways their personalities could not have been more different --- she was ebullient where he was glum, relaxed where he was awkward, garrulous where he was _______ --- they were surprisingly well suited.


We see clearly that pairs of opposite qualities are given - ebullient and glum, relaxed and awkward - therefore we need the opposite of garrulous (excessively talkative), and the ones mentioned in the choice list are laconic and taciturn.
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In this question, we see a few instances in which knowing the meaning of roots and other parts of the words could help.

solicitous - The suffix "ous" means "having the qualities of." So, "solicitous" means "having the qualities of soliciting" clueing us in to the meaning of "solicitous," which is "showing interest or concern."

irresolute - "ir" means "not." So, "ir + resolute" means "not resolute" or "not purposeful and determined."

taciturn - "Tacit" means "unspoken," and "taciturn" is a related word that means, basically, "not speaking."
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