What layer does a router primarily function at?

A router primarily functions at the Network Layer of the OSI (Open Systems Interconnection) model. This layer is responsible for routing packets of data between devices across different networks. Routers use IP (Internet Protocol) addresses to determine the best path for data to travel. They manage traffic directing data packets from the source to the destination and can also provide network address translation, which allows multiple devices on a local network to access the internet using a single public IP address.

In contrast, the Transport Layer is focused on providing end-to-end communication services, ensuring complete data transfer. The Data Link Layer is responsible for node-to-node data transfer and error detection but does not handle routing between different networks. The Application Layer is the topmost layer that interacts directly with user applications, managing high-level protocols like HTTP and FTP. Therefore, the Network Layer's role in determining the routing of data packets is what makes it the correct answer to the question regarding the primary function of a router.