Just analyze binary representation of third octet of given IP addresses:
128.56.24.0 = 128.56.00011000.0
128.56.25.0 = 128.56.00011001.0
128.56.26.0 = 128.56.00011010.0
128.56.27.0 = 128.56.00011011.0
It is clear that first 22 bits are common for all.
The answer is C) 128.56.24.0/22