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In the OSI model, each layer has a specific Protocol Data Unit (PDU) that it uses for communication. The physical layer is the first layer of the OSI model and deals directly with the transmission of raw binary data over physical media. The PDU for the physical layer is referred to as a "bit."

This is because the primary function of the physical layer is to transmit individual bits of data as electrical signals, light pulses, or radio waves, depending on the medium being used. Each bit represents the most basic unit of data in computing and telecommunications, which aligns perfectly with the responsibilities of the physical layer in terms of encoding and sending these signals across various connections.

The other terms, such as cells, frames, and packets, are associated with higher layers in the OSI model. For example, frames are the PDUs at the data link layer, packets at the network layer, and cells are specialized data formats used in certain networking technologies like ATM (Asynchronous Transfer Mode). Thus, these options do not reflect the role of the physical layer, reinforcing why "bit" is the correct answer for the PDU at this layer.