assign'g ip address to an interface.

Two Routers (R1 and R2) connected using serial s0/0 and s0/1. the Team received an ip address 192.168.151.0
we need to assign the ip address to serial s0/0 interface for R1 router.
the options available are
a. 192.168.151.72/30
b. 192.168.151.73/30

Answer shows its A.
Network Id is .72
useable address is .73 and .74.
broadcast is .75
According to my knowledge b is the valid option.
am i doing something wrong. plz explain me to select the right answer.

The network addresses associated with the /30 mask are:

192.168.151.0
192.168.151.4
192.168.151.8

...
etc.
...

192.168.151.64
192.168.151.68
192.168.151.72
192.168.151.76
...
etc.

The correct network address for the /30 subnet is 192.168.151.72 so the router serial ports are 192.168.151.73 and 192.168.151.74 leaving 192.168.151.75 as the broadcast address.

Your assessment of the question is correct as the "block size" (as indicated in one of my other responses to a post that you made) is 4.