option B seems correct answer. explanation
Network mask is designed to get network id out of IP,
In this question you have given IP of network directly, but suppose if IP in question would have been like
172.16.15.20 and mask as, 255.255.0.0
then,
1. find network address by performing "logical AND" of IP and network mask as like this
172.16.15.20
&& 255.255.0.0
______________________
172.16.0.0
Number of 1's in mask represents those many bits are reserved for n/w ID
in this case n/w id is 16 bit and host ID is 16 bits.
2. After getting network IP, put 1 in all host ID bits that will give you broadcast address of that network
in this example, 172.16.255.255