Which protocol is primarily used to transmit data over the internet?

The correct choice is TCP/IP because it is the fundamental suite of protocols that governs the internet. TCP, or Transmission Control Protocol, ensures reliable transmission of data packets between devices, while IP, or Internet Protocol, determines how those packets are addressed and routed across networks. Together, they provide the necessary functionality for sending and receiving data over the internet.

While other protocols like HTTP (Hypertext Transfer Protocol), FTP (File Transfer Protocol), and SMTP (Simple Mail Transfer Protocol) are important for specific types of data transfer — such as web pages, files, and emails respectively — they operate on top of the TCP/IP architecture. Thus, they rely on TCP/IP to establish connections and transmit their data effectively. In essence, TCP/IP serves as the backbone of internet communication, making it the primary protocol used for data transmission.