Are they here yet?
a. No, they're not.
b. Yes, they're.
I was told the answer is a. Why is it not b?
Are they here yet?
a. No, they're not.
b. Yes, they're.
I was told the answer is a. Why is it not b?
Subject-aux contractions like "they're", "I'm", "we've", "they'd" and so on cannot be used as a complete clause. They must have something after, like a subject complement, a negation, a main verb, etc.:
Yes, they're here.
Yes, I'm a teacher.
No, we've eaten.
Most evenings, they'd read.
Probably because in a such categorical short sentence we need to emphasize the word that carries the meaning (which in this case is "are") by accentuating it. The full word "are", albeit monosyllabic , can be accentuated. The weak form "'re", cannot. So you need to fully pronounce the word. Think of both the examples without the "yes/no", which could be omitted (- They are! / They are not).