fruit are?

If who, what, how, or where, is a singular reference even the nouns or pronouns

preceding are plural, then how would you call sentence B

A- John is the one who is in charge of the food for the party- Correct.

B - John and Mary are the only officers who make decisions around here.

(Is it “makes” or “make”?)

I believe 'what' is the same in usage as 'who' in the sense that the verb which immediately follows it is singular.

Goodman

The approach to treat  "what" as sigular even when the preceding pronouns are plural in my opinion is incorrect. 

I see. I get your point. Grammarians treat what as a relative pronoun that has no antecedent, singular or plural. It is inclusive of the antecedent and can usually be replaced with that which, in which collocation that is the antecedent. That which is not very common and therefore may sound odd to some natives' ears but it is correct English.

I didn't believe what / that which he told me.

So, we can agree that we disagree. You consider what as plural or singular depending on the preceding word/words, and I and grammarians always consider it singular because it has no antecedent.

CB

Goodman

As written: John and Mary are the only officers who make decisions around here. And I asked if the answer should be [makes] or [make].
And you said "In your sentence of course make"

By the nature of your comment, didn't you just agree with my view?

 Yes, as far as who is concerned. What is something else. It never has an antecedent as a relative pronoun, neither singular nor plural, nothing, and that's why a singular verb is used with it. This is obvious from the singular that, which occurs in that which, which is sometimes used to replace what. 

You can of course check the grammatical behavior of relative what in any good grammar book if you don't believe me. I find it very strange that you haven't already done that. It would have saved you a lot of time wasted writing opinions instead of facts.

CB