Which of the following protocols is used as a transport protocol for Internet dial-up connections?

SNMP (Simple Network Management Protocol) is used for monitoring and managing network devices, not for establishing dial-up transport connections.

PPP is defined in RFC 1661 as a data link layer protocol specifically designed to encapsulate and transport network-layer packets over point-to-point serial connections, which is the foundation of traditional dial-up Internet access. It provides framing, error detection, and support for multiple network-layer protocols (IP, IPX), and is the direct successor to SLIP for dial-up use.

DHCP (Dynamic Host Configuration Protocol) dynamically assigns IP addresses to hosts on a network; it is not a transport protocol for dial-up links.

SMTP (Simple Mail Transfer Protocol) is an application-layer protocol used only for sending email messages between mail servers.