The most common protocol for secure VPNs is IPsec (Internet Protocol Security). IPsec provides confidentiality, integrity, and authentication for VPN traffic, typically using ESP (Encapsulating Security Payload). It is used in both site-to-site and remote access VPNs.

B. GRE encapsulates traffic but does not provide encryption.

C. BGP is a routing protocol, not a VPN technology.

D. SSH can be used for secure tunneling but is not the standard for VPN deployment.

IPsec is the industry standard because it operates at Layer 3, securing IP traffic regardless of the application, making it highly versatile.

References (CompTIA Network+ N10-009):

Domain: Network Security — VPN protocols, IPsec, ESP.