The correct answer is D. Copper, Tin and Zinc.
Gun metal is an alloy of copper, tin, and zinc. It is a strong, durable metal that is resistant to corrosion. It is often used in the manufacture of firearms, hence the name.
Iron, Zinc and Titanium is not a correct answer because it does not contain copper.
Iron and Tin is not a correct answer because it does not contain zinc.
Iron, Brass and Tin is not a correct answer because brass is an alloy of copper and zinc.
Copper, Tin and Zinc is a correct answer because it contains all three of the ingredients of gun metal.