B. I think I'll best explain it with an example: You go to buy a new car, you choose the model you like; at the shop they have a red one... but you'd like a yellow one. So you ask: «Does this car have to be red?» that is: is it actually possible to have it red only? And they answer «No, sir, not if you don't want it to be.» that is: no, if you don't want the car to be red you won't have it red. In general, such a sentence means a thing/situation is not necessarely in a determined way, it's not like that way if you don't want it to be like that. We can analize the sentence this way: Not: adverb denying a situation/thing previously described if: particle introducing an hypothesis you don't want: main hypothesis it to be: what you don't want (it: subject / to be: verb) [e.g. If you're unhappy when the weather's cold, you don't want it to be cold.]

You should imagine it like “B”. This is because this sentence is used to retort (to refute or to say it’s the opposite way) a sentence which is not true. Someone says that X is true, and you say “not if you don’t want X to be (true)”. Example: 1: It’s impossible to lose weight! 2: It’s not if you don’t want it to be. If you are really committed to losing weight, eat better and exercise more! This says: yes it’s impossible to lose weight if you just think it’s impossible and sit around complaining and don’t do anything. But if you don’t want it to be impossible, then you need to change your attitude and think that it is possible, and then it because you think it’s possible your actions will be able to be different and you can achieve the goal.

wouldn't it be more polite to say "i would like some explanation of the sentence?" :P