Which wildcard mask would be used to match all the addresses that exist within the 87.100.45.0/21 network?

A) 0.0.0.7
B) 0.0.7.255
C) 0.0.15.255
D) 0.0.31.255

B
Explanation: B) The wildcard mask required is based on the subnet mask given. In this case, the subnet mask was 255.255.248.0; the wildcard equivalent is 0.0.7.255.
The following is the math for the last two octets:
248 = 11111000
7 = 00000111
255 = 11111111
0 = 00000000
The wildcard is just the inverse of the subnet mask.