Considering the contrast in moods, the correct word to fill in the blank is 'boisterous' which indicates a state of being lively, energetic, and noisy.
In the given sentence, it's clear that they are often in contrasting moods. So, when she is sullen (downbeat and somewhat gloomy), he is likely to be in a contrasting state i.e., more upbeat and energetic. The option that best fits this contrast is B) boisterous, which implies someone who is exuberant, lively, or noisy.
The explanation for this is based on the pattern established in the first part of the sentence. The keyword here is 'contrast'. The sentence establishes a pattern of her mood being the opposite of his. So, when she is 'sullen', we should look for a mood for him that's the opposite of 'sullen'.
Therefore, when she is sullen, he is not spiteful (which is a negativity and thus similar), not dour (which is equivalent to sullen), not inquisitive (which doesn't have an indication of mood) and not ponderous (which means slow and clumsy due to great weight); but rather, he is boisterous.
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