I believe that all three answers could be correct in your example.
I do think as you learn the language more, certain things will sound more natural simply because people use them more than others, but not because one is correct and one is incorrect.
To me, personally, option "c" sounds awkward because I don't know what is coming before it. But for example, if the sentence before your exercise said, "God is powerful and loves us," then I would use "the reason why" and think it sounded fine. But without knowing what comes before this, it sounds a little bit awkward because the reason is coming at the end of the sentence.
(Remember, though, that this is just my personal preference because it sounds awkward to me. Other English speakers from different places or with other preferences might have no problem with it ever.)