Answer:
B. They connect a dependent clause with an independent clause.
Step-by-step explanation:
Dependent clauses, as the name suggests, don't convey the full idea or thought and they can't stand on their own as a complete sentence.
The only way they can be meaningful is when they are connected to independent clause. This connection is mediated via subordinating conjunctions.
Subordinating conjunctions help us link ideas from dependent and independent clause into a full, meaningful sentence.
Some of the subordinating conjunctions often used are because, when, although, since, if, whenever, until, unless etc.