Which pairing correctly matches an OSI layer to its PDU?

• A. Data Link layer — packets
• B. Network layer — frames
• C. Physical layer — bits
• D. Transport layer — data

Answer: (C)

In the OSI model, each layer uses a specific type of data unit called a PDU. The Physical layer deals with the actual signals on the transmission medium, so its PDU is the raw bits that are sent and received. As data moves down the stack, higher-layer PDUs get encapsulated with headers/trailers as they become the lower-layer PDUs, and then they are transmitted as bits. The Data Link layer’s PDU is a frame, the Network layer’s PDU is a packet, and the Transport layer’s PDU is a segment. So pairing the Physical layer with bits perfectly matches what the layer handles on the wire. The other pairings mix up the PDU types for their respective layers: frames belong to Data Link, packets to Network, and the Transport layer uses segments rather than just data.