option A : let m=3 n=2,
push a : aaa push b : bb , now we cant pop b's from a's and so on.. So it can not be done with one stack thus it is not CFL.
Option B : Since option A proves it not CFL ,it can not be described by some regular expression because
not CFL -> not REGULAR.
Option D: Pumping lemma is a negativity test and used to say whether a language is not Regualr(pumping lemma for regular language here).
So C seems right answer although I am not getting exactly what it is.
