A rhyming couplet is two lines in a poem which rhyme and typically have the same meter, which means they have the same number of syllables.
The correct answer is D. "Her breath in mine, our fluent dipping knives-/ Never closer the whole rest of our lives"
A and B don't even rhyme, and C belongs to a quatrain, not a couplet, and those two lines don't have the same meter. D meets all the requirements of a rhyming couplet.