Final answer:
IPSec isn't a tunneling protocol but is often used to secure tunnels created by protocols like PPTP, L2TP, and L2F for network security. It is designed to provide privacy, integrity, and authenticity for data over IP networks.
Step-by-step explanation:
The protocol among the options provided that isn’t a tunneling protocol but is used by tunneling protocols for network security is A. IPSec (Internet Protocol Security). IPSec is a suite of protocols designed to ensure privacy, integrity, and authenticity of the data communications over an IP network. IPSec isn't a tunneling protocol but is often used to secure tunnels created by protocols like PPTP, L2TP, and L2F for network security. It is designed to provide privacy, integrity, and authenticity for data over IP networks.
Although IPSec can operate in a tunnel mode, it is fundamentally a security protocol rather than a tunneling protocol. Tunneling protocols, such as PPTP (Point-to-Point Tunneling Protocol), L2TP (Layer 2 Tunneling Protocol), and L2F (Layer 2 Forwarding), may use IPSec to secure the tunnel they create for data transmission.