Layer 1 of the Open Systems Interconnection (OSI) model is,

The correct answer is A, Physical Layer. The OSI model has seven layers and they are numbered from the bottom upwards, so Layer 1 is the physical layer, the one that actually carries bits over a copper cable, an optical fibre or a radio link. It fixes voltages, connectors, pin layouts, cable types and data rates, and it has nothing to do with the meaning of the data it carries. The full order from Layer 1 to Layer 7 is physical, data link, network, transport, session, presentation and application. B is wrong because the data link layer is Layer 2, which groups bits into frames and handles MAC addresses and error detection on one link. C is wrong because the transport layer is Layer 4, where TCP and UDP move data from end to end. D is wrong because the network layer is Layer 3, the layer of IP addresses and routing. Exam tip: the sentence Please Do Not Throw Sausage Pizza Away gives physical, data link, network, transport, session, presentation, application in order.