A switch populates its _____ by observing the source _____.

The correct choice is based on the fundamental operation of network switches. A switch populates its MAC address table by learning the source MAC addresses from incoming frames. This capability is essential for efficient packet forwarding within a local area network (LAN).

When a switch receives a data frame, it checks the source MAC address of that frame. It then records this address along with the corresponding port number in its MAC address table, which maps MAC addresses to the specific ports on the switch where devices are connected. This process is crucial for the switch's ability to make intelligent forwarding decisions, enabling it to send frames only to the intended recipient instead of broadcasting to all connected devices.

Other options refer to different networking concepts or components that don't apply to the specific function of a switch in this context. A routing table, for instance, pertains more to routers handling IP addresses, and packet queues deal with buffering packets in transit rather than learning about MAC addresses directly. Therefore, the focus on MAC addresses aligns perfectly with the operation of switches within an Ethernet network.