TCP is the most commonly used protocol in which layer?

TCP, or Transmission Control Protocol, is employed primarily in the Transport layer of the OSI model. This layer is responsible for providing end-to-end communication services for applications. TCP is known for its ability to ensure reliable transmission of data packets between devices by establishing a connection and managing data flow. It achieves this reliability through mechanisms such as error detection, acknowledgment of received packets, and retransmission of lost packets.

In contrast, the other layers serve different functions. The Application layer focuses on interface protocols that directly support application software. The Network layer is responsible for routing and forwarding data across the network, while the Data Link layer deals with the physical addressing and access control to the physical media over which the data is transmitted. Therefore, TCP's role in enabling reliable communication makes the Transport layer its correct classification.