A problem which is in P, is also in NP- so, A is false. If problem can be solved deterministically in Polynomial time, then also we can't comment anything about P=NP, we just put this problem in P. So, B also false. C is TRUE because that is the definition of NP-complete.
D is false because all NP problems are not only decidable but decidable in polynomial time using a non-deterministic Turing machine.