Subnet Mask = 1's (Network ID + Subnet ID) + 0's (Host ID)
A) 255.255.128.0, 2 :
11111111.11111111.10000000.00000000
having 2 subnets: means last one 1's is subnet ID
No of hosts in each subnet : 2(no of zeros) -2 = 215 -2 = 32766 (class B network)
B) 255.255.128.0, 512:
11111111.11111111.10000000.00000000
having 512 subnets: means last nine 1's is subnet ID
No of hosts in each subnet : 2(no of zeros) -2 = 215 -2 = 32766 (class A network)
C) 255.255.0.0, 1 :
11111111.11111111.00000000.00000000
having 1 subnets: means network is not subnetted.
No of hosts : 2(no of zeros) -2 = 216 -2 = 65534
Answer is D, A or B both satisfying.