All compound pronouns, yes even "everyone", are grammatically singular - the number of the postcedent (in this case, the "him" in "you help him") is not a reliable test to chose between them. Worse, "singular they" has always been a popular (in the sense of "what the people use") alternative to "generic he", so even if the postcedent had been "them", you still wouldn't be able to choose.
Aside: the reason you've gotten an answer and several comments that suggest "anyone" is plural is because singular they, although always popular, is currently deprecated by the establishment and has been for at least two hundred years. Segregating the compound pronouns like this is a misguided attempt to rationalize singular they.
The correct answer is either (a) or (c), they are both grammatical and near perfect synonyms. "Anyone", as the negative polarity alternative (meaning it is used with "not" instead of "someone"), is slightly more urgent, and what I would use, but both options are correct.