In general, any class C address is /24 or 255.255.255.0. Only supernetted class C address that are consolidated can be 255.255.240.0. 192.168.0.0 can actually have a mask of 255.255.0.0 if you wanted to. It's just that 255.255.255.0 is what's commonly used.

If that was a real question, it sucks. It's too vague and it gives multiple valid answers. You just have to pick the "best" answer.