Answer: B. Solid carbon steel
Answer Analysis:
In the training base, the welding wire is made of solid carbon steel.
Solid carbon steel welding wire is a common type of welding wire, mainly used to weld carbon steel and low alloy steel.
Flux-cored wire, aluminum, copper wire can also be used for welding, but may not be commonly used in training bases. Therefore, the answer is B, solid carbon steel.