Which protocol implements PPP over Ethernet to connect an entire network to the Internet?

Point-to-Point over Ethernet (PPPoE) is the correct answer because it is specifically designed to encapsulate Point-to-Point Protocol (PPP) frames inside Ethernet frames. This allows for the establishment of a direct connection from a user's computer or network device to an Internet Service Provider (ISP) over an Ethernet network. PPPoE is commonly used in broadband connections, such as DSL, to deliver Internet access by using the benefits of PPP, which include user authentication and data encryption.

In contrast, the other protocols listed serve entirely different functions. Simple Mail Transfer Protocol (SMTP) is used for sending emails, while Post Office Protocol (POP) is used to retrieve emails from a server, and Network News Transfer Protocol (NNTP) is utilized for reading and posting news articles. None of these protocols interface directly with Ethernet to provide Internet connectivity like PPPoE does.