Answer:
A. Chris and Molly were delighted to see the whole family gathered in the living room; the group was waiting to congratulate the couple.
Step-by-step explanation:
This sentence is the one that uses a semicolon correctly because it consists of two independent clauses, that is, clauses that express complete thoughts on their own, whose ideas are closely related to each other and that are not joined by a coordinating conjunction like so, and, or, but, or others. One of the functions of semicolons is to link two independent clauses with related ideas that are not otherwise joined by another word.