Which of the following devices operates at the Physical Layer of the OSI model?

The hub operates at the Physical Layer of the OSI model, which is the lowest layer of the model. Its primary function is to facilitate the transmission of raw data bits over a physical medium, such as a cable. Hubs transmit incoming signals to all ports, allowing devices connected to it to communicate with each other. They do not analyze or manage the data being sent—this means they do not make decisions based on the type of data or address where the data is supposed to go.

In contrast, routers operate at the Network Layer, making routing decisions based on IP addresses. Switches, while often mistakenly thought to operate at the Physical Layer, actually operate at the Data Link Layer, where they can manage data packets by making forwarding decisions based on MAC addresses. Firewalls serve a security function and typically operate at different layers of the OSI model, often at the Network or Transport Layers, using rules to allow or block traffic.

Thus, among the given devices, the hub is uniquely positioned at the Physical Layer, focusing solely on the transmission and reception of data bits without processing the content of the messages.