The alloy used for welding and plumbing contains

The alloy used for welding and plumbing is called solder, and it is made of lead and tin. Solder has a low melting point (around 183°C for the common 63/37 tin-lead mixture), which makes it easy to melt and use to join metals together.

Solder works well for plumbing because it can seal joints between pipes, creating a watertight connection. In electronics and metalwork, solder is used to join wires and components. The low melting point means nearby parts are not damaged by extreme heat during the joining process.

The other options are different alloys used for other purposes. Copper and tin make bronze, which is used for statues and bearings. Aluminum and copper make duralumin, which is lightweight and used in aircraft. Iron and aluminum do not form a common named alloy for welding.

Note: Modern plumbing increasingly uses lead-free solder for health reasons, but the traditional solder alloy tested in chemistry is the lead-tin combination.