What ___ them + single or plural verb?

Question

1. In the framework of Question word + preposition + object pronoun + verb,
   should the verb be singular or plural (ie grammatical number)?

For example, the plural verb "What about them elicits" effects a few links,
but "what about them elicit" nothing.

2. Here, the subject is what, but its numerosity is unknown because what can refer to one or many objects. So how can we determine the grammatical number of the verb?

3. What's this concept called?

4. What general lesson oversees/overrides this question? I want to learn about the bigger picture.

Answer 1

I think the key question is "What is the minimum number of subject(s) that can be used in the response?" Most of the time, questions like this can be answered by talking about one subject that most or all of "them" have in common. In such cases, "what" is singular.

So a conversation might go like this:

What about them elicits a response?

They are all intimate photos.

It is possible to explicitly make the "what" plural. For example:

What things about them elicit a response?

They are all brightly lit. All of the pictures of human faces show emotions. Many of the pictures have unusual compositions.