The correct answer is option A "Once they saved enough money for the trip". A subordinate clause is a sentence that, altough has a subject and a verb, it doesn't have sense by its own. Option B has sense by its own, but option A only has sense when it is attached to option B, so you need to read the whole text in order to understand it.
The subordinate clause within the sentence is option A) “Once they saved enough money for the trip.”
Subordinate clauses begin with a subordinating conjunction, in this case “once” and they do not have meaning on their own since they need the main clause to complete the idea. Subordinate clauses can be included at the beginning or at the end of the sentence. In the example provided, the clause is at the end.
