Adverb clauses answer the questions when, how, where, and why. It's easy to spot as it is usually preceded with a subordinating conjunction. Subordinating conjunctions include after, because, since, where, while, etc. Reviewing the choices:
(b) near the front of the store- near isn't a conjunction. It's an adjective
(c) wait with your friend- wait isn't a conjunction. It's a verb.
However, (a) while I finish grocery shopping is introduced by the subordinating conjunction "while". It is then the adverb clause of time.