This questions falls more under the scope of test question writing than grammar. By convention, when asking multiple-choice questions like this with singular answers and an 'all of the above' option, you phrase your question in the singular. This is because when you select 'all of the above', you're saying that all the other answers can be a correct answer to the question individually. Using the singular what is the benefit, picking 'all of the above' is equivalent to picking all of these:
What is the benefit of acknowledging your partner’s feelings according to the book? A. It shows him that you empathize with him.
What is the benefit of acknowledging your partner’s feelings according to the book? B. It helps to keep his defenses lowered.
What is the benefit of acknowledging your partner’s feelings according to the book? C. It makes him feel respected and unconditionally loved.
So even though there are multiple correct answers (and thus using what are the benefits may seem correct at first) in the context of a multiple-choice question I believe this to be best.
There's also the element that if you write the question as what are the benefits you run the risk of tipping off your question takers into selecting 'all of the above', as none of A, B and C work grammatically with the question, leaving D as the only logical choice.