English question, please help?

Since "What's this" is really "What is this" then I think you should go with "This is...." or "That is..." as the proper response.
But really, all of them are fine. :)

I teach ESL to adults and I have had lessons on this type of beginning conversation. For me, the answer is "It is..." for a very good reason. That is the way you would normally respond in a conversation, if someone asked you what is this, you would say, "It's a ball," not "This is a ball."

The teaching of this, that, these and those is a lesson unto itself, and it can be lots of fun, getting up close to one thing, and saying "this is a ball" then moving far away from it and saying "that is a ball," proceeding in the same fashion with the plural these and those.

Anyhow, this is how I would do it. It is whatever advances and teaches normal speech patterns.

Best of luck,
Lady Morgana
ESL teacher for 13 years

Lady Morgana gave you some excellent advice. "It's a . . ." is natural English conversation. To that I would add another reason for teaching "It's a . . . ." When you ask, "What's this?" you are probably standing at the front of the classroom. Your students are probably seated at their desks. Therefore, they are not standing near the object you are holding or pointing at. Saying "This is a . . . " would be incorrect from their point of view. They could say "That's a . . . ." but " It's a . . ." is a more typical response at least in the US.

I hope you are enjoying teaching ESL as much as I did.

I say both, if you only teach them one there gonna have to learn the other eventually.

both will do

This is ____. I say no to it is because if they get lazy they might start writing "it's" and in catholic school I learned that contractions make u sound stupid. Also, since the question is what is this__, it sounds more proper to respond this is__.