In a question like this, you need to look for an interesting octet where you can
combine networks. In this example, the third octet has all our subnets so we just
need to find our block size now. If we used a block of 8 starting at 172.16.0.0/19,
then we cover 172.16.0.0 through 172.16.7.255. However, if we used 172.16.0.0/20,
then we’d cover a block of 16 which would be from 172.16.0.0 through
172.16.15.255, which is the best answer.