--description--

People often use contraction to make speech or writing more concise. They are can be abbreviated to form a contraction.

--questions--

Which of the following is the correct contraction for they are?

they're

their

This is a possessive adjective and is not an contraction for they are.

theres

This is not a standard contraction in English.

they

Without the apostrophe and re, this is just the pronoun without the verb.