They like banana
That You didn't mark the underlined part!
If the underlined part is they, it's who like banana?
If the underlined part is like, what do they think of banana?
What do they like?
Do they like banana? To singular
Does he/ she like bananas?
Question the words in brackets: they like (hamburgers.)
The noun in brackets is the object of like, so the question word should be what
What do they like
They like bananas and apples
() () they like?
what
do