I believe A is actually plural by implication here. Typically it would be a combination of a number of things that would result in a person thinking in a certain way, so, by implication, "What" is plural. (Of "What" could be singular by implication if it is a single thing only that is making person think in a certain way.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
jooneyWhen an interrogative pronoun 'what' is in a subject position, is it possible for a verb that follows it to be in a plural form?Not unless it's the verb to be linking to a plural - or similar linking verb or expression.
jooneyA: What are the things that bother you?Yes. That's why they all contain plurals.
B: What were the most important items on the list?
C: What seem to be the reasons for his behavior?
Do you assume a plurality of things here?