An administrator connects two devices using an Ethernet cable. The link fails to come up on either device, which setting could prevent the link from being established?

Proxy settings are an application-layer configuration that route HTTP/HTTPS traffic through an intermediary and have no effect on establishing a physical Ethernet link.

IP configuration operates at Layer 3 and only matters after the physical and data-link layers are already functional - it cannot prevent the link itself from coming up.

Link speed (and duplex) operates at Layer 1 and Layer 2 and directly controls whether two devices can negotiate a valid Ethernet connection. A mismatch in configured speed between two directly connected devices can cause the physical link to never come up. Unlike auto-negotiation failures, a hard-coded speed mismatch leaves neither end able to detect a valid signal.

DNS resolvers are an application-layer service for name resolution and play no role in establishing a physical Ethernet link between two devices.