Answer: C) subordinating conjunction.
Explanation: a subordinating conjunction is a word that joins a dependent or subordinate clause (a phrase that doesn't have complete meaning on its own) with an independent clause (it has complete meaning on its own). There are subordinating conjunctions that show a relationship between two clauses involving a transition of time or place. In this case, "after" is a subordinating conjunction joining the independent clause "Mrs. Jenkins thanked them gratefully" and the subordinate clause "After the police came to her rescue."
