Is what an adjective?

Answer 1

Yes, it can be.

What is most commonly a pronoun, as in 'What did you say?'.

But what can also be used, colloquially, as an alternative interrogative adjective to which, as in 'What horse do you think will win the race?'.

What was famously used by Shakespeare in Hamlet as a relative adjective: 'What dreams may come, when we have shuffled off this mortal coil, must give us pause.' This usage is still valid, though it is not very common. For example:

'What behaviour you get from your children will depend on the example you set them.'

That is not incorrect, but it is less idiomatic than 'The behaviour that you get from your children will depend ...'

What is also an interjection: 'What? I don't believe you!'