Which sentences are correct?

Only the last is incorrect.

Thanks, centrox. I'm sorry my question is not clear enough much as I tried to make it clear.

He is clever.
He isn't stupid.
He has eaten.
He haven't eaten.

Which sentences are correct? (Is this phrasing of the question OK, as I believe more than one sentence above is correct.)

In other words, I wonder if "Which sentences are correct" is OK or has it to be "Which is the correct sentence?"

Thanks.

Your phrasing is correct. Centrox's answer is complete. That is, if only one is incorrect, the others must be correct.

Both are correct grammatically. The second would be a poor choice, for an obvious reason.

Thanks, centrox.

What about "Which are the correct sentences"? Is this ok?

I am confused because "Who are online?" is incorrect, but "Which is online?" is fine.

Which are the correct sentences is fine. Are is for plural items. We say who is online/here/there etc because the number is indefinite and could be only one.

Thanks, centrox.