Explanation:

The correct answer is option 'C' which is IP (Internet Protocol).

Network Layer:
The network layer is the third layer in the TCP/IP protocol suite. It is responsible for the logical transmission of data between network entities. The network layer provides services for routing and forwarding of packets across different networks.

TCP/IP Protocol Suite:
The TCP/IP protocol suite is a collection of protocols that are used for communication over the internet. It consists of four layers: the network interface layer, the internet layer, the transport layer, and the application layer.

Protocols at the Network Layer:
At the network layer, the protocol defined by TCP/IP is IP (Internet Protocol). The IP protocol provides the addressing and routing functions required for the delivery of packets across different networks. It is responsible for breaking the data into packets, adding the necessary addressing information to the packets, and ensuring the proper delivery of packets to their destination.

TCP:
TCP (Transmission Control Protocol) is a transport layer protocol that provides reliable, connection-oriented communication between applications. It operates above the network layer and uses IP for addressing and routing. TCP ensures that the data is delivered in the correct order and without any loss or corruption.

UDP:
UDP (User Datagram Protocol) is another transport layer protocol that provides unreliable, connectionless communication between applications. It is a simpler and faster protocol compared to TCP but does not guarantee the reliable delivery of data.

Conclusion:
In summary, at the network layer, the protocol defined by TCP/IP is IP (Internet Protocol). TCP and UDP are transport layer protocols that operate above the network layer and use IP for addressing and routing.