Don't go cross-eyed on this one! Their, there, they're all sound alike, and our brains can easily interchange them when writing, but watch for that trap!
To add to the confusion, we're accustomed to using an apostrophe to make words possessive, such as "Terry's coffee is great." We can then mentally justify "They're coffee is great," even though we really know that's not right if we stop to think about it.
The correct answer is "b) They’re going to meet us there? Their coffee is great."
The trick is to remember that "they're" is a contraction, not a possessive: that apostrophe is taking the place of a missing letter (from "they are") and not serving the same purpose as the apostrophe-S possessive in "Terry's."
"Their" is part of the same mental trap as "it's/its," but pronouns are powerful and can be possessive without using punctuation tools like mere nouns.