Which of the following protocols implements VPN using IPSec?

SLIP (Serial Line Internet Protocol) is a legacy point-to-point protocol that provides no encryption or VPN capability whatsoever.

PPP (Point-to-Point Protocol) is a data-link layer protocol used for direct connections; it does not implement IPSec or provide VPN tunneling on its own.

L2TP does not provide encryption on its own; it relies on IPSec (specifically IPSec ESP in transport mode) to supply confidentiality, integrity, and authentication for the tunnel. This pairing, known as L2TP/IPSec, is a widely standardized VPN solution defined in RFC 3193 and is natively supported in Windows, iOS, and other platforms. IPSec handles key exchange (via IKE) and packet-level encryption, while L2TP provides the tunneling framework.

PPTP (Point-to-Point Tunneling Protocol) creates VPN tunnels but uses MPPE (Microsoft Point-to-Point Encryption) for security, not IPSec.