This is a fairly tricky question. I don't recommend diagramming for questions like this. The question itself doesn't revolve around the conditionality, rather it is just present. As you continue to practice, you will develop a sense of when to diagram and when to simply analyze the stimulus and answer choices as they are.
Let's discuss C first, even if love can refer to something other than feelings, this does not validate the conclusion that love should not be understood to refer feelings in marital vows. There are many words in English that have multiple meanings, and we can interpret them as we see fit.
I would say that answer choice B is an appealing trap answer. However, this answer choice is telling us about promises that shouldn't be made. But the argument is discussing how to interpret promises that are already made. This is an important distinction.
Answer choice D tells us that "promises should not be interpreted in such a way that they make no sense". If referring to love as a feeling in marital vows makes no sense, then this validates the conclusion that we shouldn't interpret love as a feeling for martial vows.