The answer is: A.) Once they saved enough money for the trip.
Subordinate or dependent clauses cannot stand on their own because they do not express complete thoughts, so they need to join a main clause in order to make sense. They begin with a subordinate conjunction, such as although, since or once, or a relative pronoun, like who, which or that.
In the example sentence, "Allison and Andy want to travel to India someday" is the main clause because it is a full idea that can stand alone.
