Which protocol operates at the Transport Layer to ensure reliable transmission?

The protocol that operates at the Transport Layer to ensure reliable transmission is TCP, which stands for Transmission Control Protocol. TCP is specifically designed for reliable communication between devices on a network. It achieves this by establishing a connection before data is sent, ensuring that data packets are delivered in order, and confirming receipt through acknowledgments. If a packet is lost during transmission, TCP will retransmit it, thus guaranteeing the integrity and reliability of the data being communicated.

In contrast, other protocols listed do not provide the same level of reliability. For instance, UDP (User Datagram Protocol) is also a Transport Layer protocol, but it does not ensure reliability, as it sends data without establishing a connection or confirming delivery. ICMP (Internet Control Message Protocol) operates at the Network Layer and is mainly used for error messages and diagnostics, rather than for ensuring reliable data transmission. IP (Internet Protocol) functions at the Network Layer as well and is responsible for routing packets to their destinations without any guarantees of delivery or order. Therefore, TCP's capabilities for reliable transmission make it the correct answer.