Final answer:
The widely used protocol for Internet dial-up connections is PPP (Point-to-Point Protocol).
Step-by-step explanation:
The protocol widely used today as a transport protocol for Internet dial-up connections is PPP (Point-to-Point Protocol).
PPP is a data link layer protocol that is commonly used to establish a direct connection between a user's computer and an Internet Service Provider (ISP) over a telephone line.
It provides authentication, compression, and error detection features, making it suitable for dial-up connections.