Final answer:
Bridge priority is a field with a default value of 32,768 and is the initial deciding factor when electing a root bridge in STP.
Step-by-step explanation:
The term that describes a field that has a default value of 32,768 and is the initial deciding factor when electing a root bridge is Bridge priority. Bridge priority is a numerical value assigned to a switch to determine its priority in the spanning tree. The switch with the lowest bridge priority value becomes the root bridge.
Other options mentioned in the question include:
BPDUs (Bridge Protocol Data Units): These are messages exchanged between switches to establish and maintain the spanning tree topology.
Bridge ID: This is a combination of the bridge priority and the switch's MAC address.
Extended system ID: This is an optional field that can be used to extend the bridge ID beyond the MAC address.