Which layer of the OSI model is responsible for providing end-to-end communication services and ensures complete data transfer?

The Transport Layer (Layer 4) of the OSI model is responsible for ensuring end-to-end communication between devices. It handles the complete transfer of data by managing error detection, flow control, and segmentation of data into packets. Protocols such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate at this layer, with TCP providing reliable data transmission and UDP offering connectionless communication. Option A (Physical Layer) is incorrect because the Physical Layer is responsible for the transmission of raw data bits over a physical medium such as cables or radio frequencies. Option B (Data Link Layer) is incorrect because the Data Link Layer handles node-to-node communication, including error detection and frame synchronization, but not end-to-end communication. Option D (Network Layer) is incorrect because the Network Layer handles logical addressing (IP addresses) and routing, but does not ensure data integrity or reliability. Option E (Application Layer) is incorrect because the Application Layer provides network services directly to user applications but does not manage the transmission or reliability of the data itself.