Which of the following address types is used at Layer 3 of the OSI model?

Correct answer: IP addresses

Option A is correct because Layer 3 of the OSI model is the Network layer, and IP addresses (both IPv4 and IPv6) are the logical addressing mechanism used at this layer to identify source and destination hosts across different networks and to enable routing between them. Option B is incorrect because port numbers operate at Layer 4, the Transport layer, and are used by TCP and UDP to identify specific applications or services on a host rather than network-level addressing. Option C is incorrect because MAC addresses are Layer 2, Data Link layer addresses that identify devices on the same local network segment and are used by Ethernet for frame delivery within a broadcast domain. Option D is incorrect because application identifiers are an abstract concept associated with Layer 7, the Application layer, and do not perform the network-level addressing function defined for Layer 3.