Which protocol operates at the Transport layer of the OSI model?

The protocol that operates at the Transport layer of the OSI model is TCP (Transmission Control Protocol). The Transport layer, which is Layer 4 of the OSI model, is responsible for providing reliable data transfer services to the upper layers, enabling processes running on different hosts to communicate effectively. TCP plays a crucial role in ensuring that data is delivered in sequence and without errors by implementing error-checking, flow control, and connection management.

In contrast, the other choices refer to protocols that operate at different layers of the OSI model. IP (Internet Protocol) operates at the Network layer, which is responsible for routing packets across networks. ARP (Address Resolution Protocol) functions at the Data Link layer, helping map IP addresses to MAC addresses within a local area network. Ethernet is also a protocol that operates at the Data Link layer, managing the frame encapsulation for physical transmission over network mediums.

Therefore, TCP is the correct answer as it directly relates to the Transport layer's functionality in ensuring reliable, orderly communication between applications over a network.