This sentence has unequal ideas joined by a subordination conjunction.
Since the ideas in the sentence imply causation (the broken pulleys caused the inability to raise the flag) it means that the ideas are unequal. It is also necessary to join them with a subordinating conjunction, which show the causality within the sentence. In this case that subordination conjunction is "because".
I just learned prep. phrases like 3 months ago but i think it is B.
