Final answer:
The incorrect definition related to arson detection is C: Trailers, which actually refers to materials used to spread fire, not materials left behind after a fire.
Step-by-step explanation:
The definition in relation to arson detection that is incorrect is C: Trailers. The term 'trailers' in the context of arson refers to materials used to spread a fire from one area to another, not material left behind in the wake of a fire path. The correct definitions for arson-related terms are A: Plants, which are materials arranged to start and feed the initial fire; B: Ignition Device, which is a mechanical, chemical, or electrical means used to initiate a fire; and D: Protected area, which is the clean or unburned area left by stock, furniture, or contents.