Choose the right word to complete the dramatic cat owner's warning to a guest.

You ___ sit on that velvet armchair. Sir Fluffington has claimed it as his throne, and he does not share.

The correct answer is can't.

We use can't (cannot) to deny permission in the present. "Couldn't" refers to the past, and "aren't" or "haven't" do not fit grammatically with the base verb "sit."