The question asks at which OSI layer monitoring occurs when capturing traffic to and from a router. A router is fundamentally a Network Layer (Layer 3) device. Its primary function is to forward data packets between different computer networks by making decisions based on the logical addresses (IP addresses) contained within the packet headers. When an investigator captures traffic at a router, they are observing this inter-network forwarding process, which is the core responsibility of the Network Layer. While the captured data (e.g., in Ethereal/Wireshark) will display information from all layers, the specific activity of routing is a Layer 3 function.

B. Transport: The Transport Layer (Layer 4) manages end-to-end communication and data integrity (e.g., TCP/UDP). Routers are generally unaware of and do not operate on this layer's information.
C. Data Link: The Data Link Layer (Layer 2) handles node-to-node data transfer on the same local network using physical addresses (MAC addresses). Switches primarily operate here; routers use it but function above it.
D. Session: The Session Layer (Layer 5) establishes, manages, and terminates connections between applications. Routers are transparent to this layer and do not use its information for forwarding.

1. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). In Chapter 4, Section 4.4, "The Network Layer: Data Plane," the text states, "The primary role of the network layer is to move packets from a sending host to a receiving host. To do so, two important network-layer functions can be identified... forwarding... and routing." This section explicitly defines the router's core functions as belonging to the Network Layer.

2. MIT OpenCourseWare. (2018). 6.033 Computer System Engineering, Spring 2018. In Lecture 11 Notes, Section 11.2, "Internet Protocol (IP)," it is stated: "The network layer provides host-to-host connectivity... Routers implement the network layer." This directly associates the function of a router with the Network Layer.

3. Cisco. (2022). Internetworking Basics. In the section "OSI Model Layers and Information Exchange," the description for the Network Layer states: "The Network layer provides routing and related functions that enable multiple data links to be combined into an internetwork... The router is the classic Network layer device." This official vendor documentation confirms the router's position at Layer 3.