The given options include "multimeter," "soldering iron," "welding iron," and "spanner." Out of these options, the correct answer is "soldering iron." This is because a soldering iron is a tool used in soldering, which is a process of joining two or more metal components together using a filler metal. It is commonly used in electronics and plumbing work. The other options, such as "multimeter," "welding iron," and "spanner," do not serve the same purpose as a soldering iron.