Find the subnet ID from the IP address 10.75.20.4/17

(A) 10.75.5.0
(B) 10.75.0.0
(C) 10.75.20.0
(D) 10.0.0.0

10.75.0.0 is the correct answer.

IP address = 10.75.20.4/17
Subnet mask = 255.255.128.0
The subnet mask can be found from the prefix length /17.
The subnet mask is 255.255.128.0, so the network ID multiplier is 256 – 128 = 128
The possible subnet IDs are 10.75.0.0 and 10.75.128.0.
The network 10.75.0.0 is it as the address of 10.75.20.4 belongs to that network IP range.
The IP address range of the network is 10.75.0.0 – 10.75.127.255 (the next address 10.75.128.0 is the next subnet ID). The subnet ID for 10.75.20.4/17 is 10.75.0.0