Explanation:

- Alnico: This is an alloy composed mainly of aluminum, nickel, and cobalt. It's primarily used to make permanent magnets.

- Solder: This alloy commonly consists of lead and tin. It's used for joining metal parts together.

- Nickel: Nickel itself is a metal, not an alloy. It often forms part of other alloys but isn't mixed with lead typically.

- Constantan: This is an alloy of copper and nickel. It's known for its ability to maintain constant resistance with changes in temperature.

Solder is the correct answer as it contains lead.