What is the name of the segments into which a router can divide a physical network?

The correct choice, subnets, refers to the logical divisions of a larger network created by a router to improve performance and manageability. Subnetting allows the segmentation of a network into smaller, more efficient parts, making it easier to manage traffic, isolate network segments for security purposes, and optimize network performance. Each subnet operates as a distinct network segment, enhancing the overall structure and functionality of the broader physical network.

VLANs, or Virtual Local Area Networks, are related in that they also segment networks, but they do so at the data link layer, allowing multiple networks to coexist on the same physical infrastructure. Domains typically refer to administrative boundaries in network or security contexts, while clusters are more relevant in computing for grouping network resources. Therefore, the term that best represents the segments created by a router in a physical network is indeed subnets.