Final answer:
PPTP (Point-to-Point Tunneling Protocol) is the VPN protocol that is no longer considered secure due to vulnerabilities and weak encryption.
Step-by-step explanation:
Among the VPN protocols mentioned, PPTP (Point-to-Point Tunneling Protocol) is the one that is no longer considered secure. PPTP has been widely used because it is easy to set up and is supported across various operating systems. However, over time, vulnerabilities have been discovered that have compromised its security.
Cryptographic issues and the use of weak encryption have made it susceptible to attacks. As a result, many organizations have moved away from using PPTP in favor of more secure protocols like IPsec (Internet Protocol Security) and TLS (Transport Layer Security), which are more robust and provide better encryption.