CIDR general notation format:$a.b.c.d/n$ where n is the number of bits that are presented in network id.
- NID+SID= n bits
- HID=(32-n) bits
Subnet mask: it is $32$ bit number in which the number of 1’s indicates network id and subnet id, number of 0’s indicates host id part.
- NID+SID=all 1’s
- HID=all 0’s
Given IP address is $172.26.17.1/25$, here /25 represents the first 25 bits are all 1’s. that is
$172.26.17.1$ can be represented in binary form:
$11111111.11111111.11111111.10000000$, last $7$ 0’s represented as HID.
Therefore SM would be: $255.255.255.128$
So option $(D)$ is correct.