Final answer:
The correct practice when mounting accessories to the heat-treated frame of a truck is to never drill or weld on the flange of the frame rail.
Step-by-step explanation:
The correct practice when mounting accessories to the heat-treated frame of a truck is option c) Never drill or weld on the flange of the frame rail. Drilling or welding on the flange can weaken the structural integrity of the frame rail and compromise the safety of the truck. It is important to follow manufacturer guidelines and use approved mounting methods to ensure the accessories are securely and safely attached to the frame.