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

SMTP is an application-layer protocol for sending email and has no role in establishing dial-up transport connections.

SNMP is a network management protocol used to monitor and manage network devices, not to establish dial-up connections.

DHCP is used to dynamically assign IP addresses to hosts and operates after a connection is already established.

PPP operates at the data link layer and provides framing, error detection, and authentication (via PAP/CHAP) for serial connections such as dial-up. It is the foundational transport protocol that ISPs have long used to establish dial-up Internet sessions. Without PPP, raw dial-up connections lack the framing and negotiation needed to carry IP traffic.