The correct answer to the question is option 2, Zinc. Zinc is commonly used to protect steel and iron from rusting because it has a higher reactivity than iron. This means that it oxidizes more readily than iron, forming a protective layer of zinc oxide on the surface of the metal. This layer acts as a barrier, preventing oxygen and water from coming into contact with the iron or steel and causing rust.
Option 1, Magnesium, is not the correct answer because while magnesium is also a reactive metal, it is not commonly used for the purpose of protecting steel and iron from rusting.
Option 3, Aluminium, is also not the correct answer. While aluminium does have some rust-resistant properties, it is not as effective as zinc in protecting steel and iron from rusting.
Option 4, Lead, is also not the correct answer. Lead is a relatively inert metal and is not commonly used for rust prevention.
In conclusion, to protect steel and iron from rusting, a thin layer of zinc is applied because of its higher reactivity and its ability to form a protective oxide layer.