since it is class B address we have 16 bits for net & 16 bits for host id
to accommodate 600 host per subnet we need 10 bits (2^10= 1024 as 2^9 =512 will not hold in this scenario)
therefore we need to make rightmost 10 bits as 0 for mask and rest 22 will be as 1 which is nothing but
11111111.11111111.11111100.00000000
255.255.252.0
therefore option C