Ans : C
1) Z could be -ve thus => insuff
2) X*Y could be prime => insuff
X*Y*Z = -ve number
and since X=-Y, X*X can never be prime
Ans NO => Suff
You move in the right direction, either set is not sufficient. Combine -- the product is –X^2, a negative number can NEVER be a prime! If X=0, then tha product is 0, which is not prime either. Thus, C.