What does it mean for a problem to be NP$-$complete?

a$.$ There is no efficient $($polynomial time$)$ algorithm for the problem

b$.$ The problem is easy to solve in parallel

c$.$ The problem is in NP and every problem in NP can be reduced to this problem in polynomial tine

d$.$ The problem is in NP and this problem can be reduced to every other problem in NP in polynomial time