the key word is "the closet" so the pronoun you need is "it" A) Clean it out.

They are both correct. but you'd more than likely, clean out the closet.. you could also clean them (the toys) out.

I would say a) clean it out sounds more natural to me. "It" in this case is the closet, because that's the subject of the previous sentence. The person is asking to clean out the closet so that there won't be anymore old toys.

Both make sense, and would be perfectly normal sounding to use in a conversation, however... "Clean it out" is a little more natural, and would imply that you are going to clean out the ENTIRE cupboard, not just the toys (maybe there are some books in there too?) "Clean them out" means that you will only be taking the toys out (and not the books, if there are any)

clean it off, clean it up both apply you cant use word "them" because the subject here is in singular form