Which of the following dotted-decimal notation (DDN) is equivalent of /17.

(A) 255.255.192.0
(B) 255.128.0.0
(C) 255.255.128.0
(D) 255.255.224.0

255.255.128.0 is the correct answer.

The /17 is the equivalent of the mask that in binary has 17 binary 1s.
To convert that to DDN format, write down all the binary 1s (17 in this case), followed by binary 0s for the remainder of the 32-bit mask.
Then take 8 bits at a time and convert from binary to decimal.
Using the /17 mask in this question, the binary mask is 11111111.11111111.10000000.00000000. Each of the first two octets is all binary 1s, so each converts to 255.
The third octet, 10000000, converts to 128, for a DDN mask of 255.255.128.0.