Question

Which sentence contains an adverb clause?

Answer 1

An adverb clause has essentially the same function as an adverb. "When" in c is key to it being an adverb clause.

Answer 2

An adverb clause is a subtype of subordinate/dependent clauses, meaning that it cannot exist on its own, outside of the main, independent clause. In these examples, there are several dependent clauses, in A, C, and D, however, only in C there is an adverb clause 'when we go to the store'. 
In the other examples, there are only adjective clauses.
So, the correct answer is C.