Correct! The answer here is c.
Many students, having heard so many times that using the word "because" in a sentence is "bad" or wrong might be fooled with this one. Bottom line is that the choice with the word "because" is the best one because (there's that word again) it is the proper connecting word and results in much smoother and simpler sentence construction. Words like "because," "on account of," "by," and "being as" create dependent clauses. "Because" also works best for this example since it is more concise (note that with one word you get the same meaning as the others) and has the best "cause and effect" results. Below find some website links to practice the use of dependent clauses.