Preposterousness is the noun: the quality of being completely unreasonable or silly. A writer might talk about the preposterousness of a plan, meaning the silliness itself, named as a thing. It is a long, heavy word. Though actually, almost nobody needs it in speech, and even on paper it can sound like you are showing off the length. In class, how preposterous it is or just how silly that is will usually serve you better. If you do use it, keep it for serious or gently humorous formal writing, not for a message to a friend. The meaning is the same as preposterous; only the job in the sentence has changed.