Student or Learner
I was told that we can use "What's this?" and "What's that?", but we cannot use "What's it?" What is the reason for this?
I don't think there's a grammatical logic behind it as we can say What's s/he?, so it a collocation or habit. I think saying we cannot is turning a pattern into an absolute rule. I'd refine it to say that Most people use "What is it?" so follow their lead.