When forming a piece of metal, what type of tool would you use?

Using a ball peen hammer is appropriate when forming a piece of metal because it is specifically designed for metalworking. The rounded end, or "peen," of the hammer is effective for shaping and forming metal surfaces, allowing for indentation and precision work in creating designs or altering the shape of the metal. Additionally, the flat face is used for striking, which helps to flatten or smooth metal surfaces effectively.

The other tools mentioned are less suited for metal forming. A claw hammer is primarily designed for driving nails and removing them, which does not apply to metal shaping. A rubber mallet is used for softer materials and may not provide the necessary force without damaging the metal. Finally, a flat hammer is not as versatile as a ball peen hammer for the specific purposes of metal forming, since it lacks the additional functionality provided by the peen.