Step 1: Understand the alloys of iron.
Invar is an alloy of iron and nickel, known for its low coefficient of thermal expansion.
It is used in precision instruments.
Step 2: Identify the correct alloy.
Brass is an alloy of copper and zinc, vitallium is a cobalt-based alloy, and solder is an alloy of lead and tin, none of which are alloys of iron.