What information within a data packet does a router use, by default, to make forwarding decisions?

A router uses the destination MAC address, by default, to make forwarding decision.

MAC (Media Access Control) address is a unique identifier attached to networking equipment such as routers, Ethernet cards and other devices. It is a device hardware address. Every device connected to a local area network has a unique MAC address assign to it. MAC addresses are layer 2 addresses in the frame header.

The layer - 2 frame encapsulates the layer 3 IP packet. Unlike IP address that changes any time you connect to a different network, MAC address can still identify the device.