正解:C
In CEH v13 Module 14: Cryptography, IPSec (Internet Protocol Security) is defined as a framework used to establish secure communication channels over IP networks, particularly in VPNs.
IPSec supports encryption, authentication, and integrity.
Operates in Tunnel Mode (VPNs) or Transport Mode (End-to-End).
Core protocols: AH (Authentication Header) and ESP (Encapsulating Security Payload).
Option Analysis:
A). PEM: Privacy Enhanced Mail (email encryption).
B). PPP: Point-to-Point Protocol (data link layer, not encryption).
C). IPSEC: VPN encryption protocol.
D). SET: Secure Electronic Transaction (used in payment systems, obsolete).
Reference:
Module 14 - IPSec Overview in VPN Security
CEH eBook: Protocols Used in VPNs - IPSec, L2TP, PPTP