A security administrator is creating a new firewall object for a device with IP address...

A /25 subnet mask means 25 bits are reserved for the network portion, leaving 7 bits for host addresses. In dotted decimal, that is:

11111111.11111111.11111111.10000000

Decimal equivalent: 255.255.255.128

A. 255.255.254.0 corresponds to /23.

B. 255.255.255.1 is invalid as a subnet mask.

D. 255.255.255.192 corresponds to /26.

Thus, the correct subnet mask for a /25 network is 255.255.255.128.

References (CompTIA Network+ N10-009):

Domain: Networking Concepts — Subnetting, CIDR notation, dotted decimal.