This is CIDR or supernetting. They want you to combine three class C network, so just write them out in binary
00001100 = 12 00001101 = 13 00001110 = 14------
11111100 is the common mask, so 255.255.252.0 is the best answer, but you have also combined 196.56.15.0 by doing this. Not a good question, IMHO.Doan