NoAgenda, Actually B is possible; however, A is better because more accurate. Circumstances (plural) can refer to the logical surroundings, i.e., the time, place, manner, cause, occasion, etc. in which some action takes place. -Due to circumstances beyond our control the flight has been cancelled. Circumstance (singular) refers to one of the above factors. -It was late but he overlooked that circumstance.

You use "moment" here because he is saying that right now he is too busy to help, another time he would be able to. However "circumstance" here would mean more the situation for which you are asking for help. He says that he can't help because he's busy at the moment, not because you shouldn't be asking him. Example: "This is not the right circumstance to ask for my help. You should ask a classmate for help with this sort of thing." "This is not the right circumstance to ask for my help. You should look through the book and try to solve it yourself. Only after you have tried everything else should you come to me for help." "This is not the right moment to ask for my help. I have a meeting in 5 minutes, but I can help you later."