Step 1: Define network edge vs. network core.
The network is conceptually divided into the edge and the core.
Network Edge: Contains the end systems that users interact with, which run applications (e.g., PCs, servers, smartphones).
Network Core: Contains the interconnected routers and switches that forward data through the network.
Step 2: Classify the devices.
(B) PC, (C) Smartphone, (D) Server are all end systems, placing them at the network edge.
(A) Switch is a device that operates within the network core to connect edge devices and forward traffic between them.
Therefore, a switch is not a network edge device.