Which OSI Layer is concerned with process-to-process communication?

The Transport Layer is concerned with process-to-process communication, which involves the delivery of messages between applications or services on different devices. This layer is responsible for ensuring that data is transferred error-free, in sequence, and without losses or duplications. It provides functionalities such as flow control, segmentation, and error correction, allowing for reliable communication between processes running on different hosts.

The protocols that operate at this layer, such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol), directly handle the transportation of data segments between applications, ensuring that they are properly compiled and delivered to the correct destination. This is fundamental for enabling applications like web browsers and email clients to communicate effectively over a network.

In contrast, the Network Layer focuses on the delivery of packets across networks and handling routing, while the Data Link Layer is concerned with node-to-node communication and managing physical addressing within a local area. The Physical Layer deals with the hardware transmission of raw bitstreams over a physical medium. Thus, the emphasis of the Transport Layer on end-to-end communication makes it the correct choice for this question.