Answer:
A. They decided to meet where the band was performing.
Step-by-step explanation:
The only sentence that has a clause that's necessary to the sense/comprehensibility of the meaning is the first one. This clause is "where the band was performing."
Restrictive clauses give essential information in a sentence. The sentence does not make sense without the clause.
In this case "where the band was performing" is the restrictive clause.