Final answer:
PPTP is the unsuitable protocol for WAN VPN connections due to its weak security features. IPSec is recommended for secure VPN connections. PPP and L2TP are other protocols used with VPNs but do not offer encryption without IPSec.
Step-by-step explanation:
The protocol that is unsuitable for WAN VPN connections among the four options listed is PPTP (Point-to-Point Tunneling Protocol). PPTP is one of the oldest VPN protocols and is widely regarded as obsolete due to its weak security features. Modern VPN setups typically use more secure protocols such as IPSec (Internet Protocol Security), which is designed for securing Internet Protocol communications by authenticating and encrypting each IP packet in a data stream.
PPP (Point-to-Point Protocol) is a data link protocol that can establish a direct connection between two networking nodes, which can also be used in establishing VPN connections, though it is not directly a VPN protocol.
L2TP (Layer 2 Tunneling Protocol) is often used combined with IPSec to provide security. L2TP by itself does not offer encryption; it simply creates a tunnel, while IPSec handles encryption, channel security, and data integrity.