The correct answer is C. Metal type and thickness.
The pre-heating temperature for welding is primarily determined by the type of metal being welded and its thickness. Different metals have varying thermal properties and require specific pre-heating temperatures to prevent issues like cracking. Thicker materials typically require higher pre-heating temperatures to ensure proper heat distribution and minimize stress.
While other factors like welding machine power and ambient temperature can influence the welding process, they are not the primary determinants of pre-heating temperature. Welder's experience level does not directly influence the necessary pre-heating temperature, although an experienced welder may understand how to adjust for specific conditions effectively.