Which of the following are suitable protocols for securing VPN connections at the lower layers of the OSI model?

S/MIME is for securing email (Application Layer), and SSH (Secure Shell) is for secure remote access (Application Layer), not typically for general VPN tunnel creation at lower OSI layers.

TLS (Transport Layer Security) and its predecessor SSL operate at the Transport Layer (Layer 4) and are primarily used for securing application-layer protocols like HTTPS, not typically for full network-layer VPN tunnels.

IPsec (Internet Protocol Security) operates at the Network Layer (Layer 3) of the OSI model, providing authentication and encryption for IP packets. L2TP (Layer 2 Tunneling Protocol) operates at the Data Link Layer (Layer 2) and is commonly used with IPsec to provide a secure tunnel for VPN connections by encapsulating frames within IP packets, leveraging IPsec for security.

PKCS#10 is a certificate request standard, and X.509 is a certificate format standard; neither are protocols for establishing and securing VPN tunnels.