The correct answer is (a) Pig iron.
Pig iron is a type of iron that is produced by smelting iron ore in a blast furnace. It is a very impure form of iron, containing about 4% carbon. Wrought iron is a type of iron that is produced by hammering and rolling molten iron. It is a very pure form of iron, containing less than 0.1% carbon. Steel is a type of iron that is produced by adding carbon to pig iron. The amount of carbon in steel can vary, but it is typically between 0.05% and 2.11%. Alloy steel is a type of steel that contains other elements, such as chromium, nickel, or manganese. These elements are added to improve the strength, toughness, or corrosion resistance of the steel.
Pig iron is the most carbon-rich of the four options, followed by steel, wrought iron, and alloy steel.