Which protocol suite performs functions of OSI layer 4?

The correct choice is TCP because it operates at Layer 4 of the OSI model, known as the Transport layer. The primary functions of this layer include providing end-to-end communication services, ensuring reliable data transfer, and managing flow control and error recovery. TCP (Transmission Control Protocol) is a connection-oriented protocol that establishes a connection between sender and receiver before data transmission begins, allowing for reliable communication through sequence numbering and acknowledgment of received packets.

In contrast, the other mentioned protocols serve different purposes and operate at different layers of the OSI model. For instance, IPX (Internetwork Packet Exchange) is primarily associated with network layer functionalities, while MAC (Media Access Control) is related to the Data Link layer, dealing with the physical addressing mechanism for network devices. CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a protocol used in the network layer for controlling access to the shared communication medium in Ethernet networks, also at the Data Link layer. Thus, TCP is distinctly the protocol that performs functions specific to the Transport layer.