Which of the following is a key transcendental principle in regards to nature?

As standardly conceived, transcendental arguments are taken to be distinctive in involving a certain sort of claim, namely that X is a necessary condition for the possibility of Y - where then, given that Y is the case, it logically follows that X must be the case too. Moreover, because these arguments are generally used to respond to skeptics who take our knowledge claims to be problematic, the Y in question is then normally taken to be some fact about us or our mental life which the skeptic can be expected to accept without question