The correct answer is C, four-step working mode.
Answer analysis: In the welding of the training base, the working mode of welding adopts a four-step working mode, that is, the four steps of preparation, assembly, welding and inspection.
The spot welding working mode is for thin plate welding, the two-step working mode is suitable for manual welding, and the special four-step working mode is based on the four-step working mode with some improvements and optimizations.
Therefore, option C, the four-step mode of operation, is the most appropriate answer.