What protocol can also be used in SSTP to establish a VPN without requiring client setup?

The correct choice is SSL, as it plays a pivotal role in establishing a secure communication channel for VPN connections, particularly through SSTP (Secure Socket Tunneling Protocol). SSTP utilizes SSL to encapsulate and encrypt the traffic that is being sent over the internet. This integration allows for a more streamlined setup for the client because SSL is widely supported and can traverse firewalls and proxies, which often allow HTTP traffic.

Using SSL for VPNs ensures that the connection is secure and that the data being transmitted is encrypted, preventing unauthorized access or eavesdropping. The protocol facilitates a secure tunnel for the VPN traffic, helping to protect the integrity and confidentiality of the data.

Other protocols mentioned in the options either do not align with the primary function of SSTP or do not inherently simplify client configuration. HTTP, for instance, lacks the encryption necessary for secure communication, while TLS is a successor to SSL that typically requires SSL as its underlying protocol. UDP, on the other hand, is a transport layer protocol primarily used for sending messages without a guarantee of delivery, making it unsuitable for establishing secure VPN connections.