Question

Both Lucy and Paul finished their work because they stayed after school.

Answer 1

Because is the correct answer. The word because is connecting the two statements together and explaining how they finished their work.

Answer 2

The word in the sentence that is a subordinating conjunction is because.

Subordinating conjunctions, or subordinators, are words or phrases that join dependent clauses to independent clauses. These conjunctions provide a transition and, moreover, indicate what type of relation is the one that connects both clauses.