Correct! The answer here is b.
This one is about style and preserving the meaning of the original
sentence with the most exactness. The original "if" statement presents
a "cause and effect" situation. "Because" is the best word to use in
the rewrite since it keeps that cause and effect relationship alive. Below
see some pointers on dependent clauses, which often create "cause and
effect" situations in writing.
Skill Practice I
Skill Practice II