What are essential things required for rust?
A. Strong acid, oxygen, an electrical current.
B. Sunlight, air, salt.
C. Water, oxygen, and metal.
D. An anode, a cathode, salt.

Hint: Rust is an iron oxide that forms when iron and oxygen react catalytically in the presence of water or air moisture. It is usually reddish-brown in colour. Rust is a form of refined iron corrosion made up of hydrous iron(III) oxides and iron(III) oxide-hydroxide.

Complete answer: As iron compounds corrode in the presence of oxygen and water, rust is formed. It's made up of a combination of iron oxides and hydroxides. Corrosion, particularly corrosion of steel, is commonly referred to as rusting.
Metallic iron, which is present naturally in the ore haematite as iron oxide, appears to revert to a similar state when exposed to air (hydrogen, oxygen, nitrogen, and other elements), as well as water. The oxidation reaction occurs when iron metal returns to an energetically favourable state, causing corrosion. When rust forms, it releases energy. The rusting process can be broken down into three stages: The formation of iron(II) ions from the metal, followed by the formation of hydroxide ions and their reaction with oxygen to form rust.
When iron and steel come into contact with water and oxygen, they rust. In salt water or acid rain, they rust more quickly. Aluminum, on the other hand, does not corrode easily due to a coating of aluminium oxide on the surface. The presence of water and oxygen is needed for iron to rust.
$
  4Fe + 3{O_2} + 2n{H_2}O \to 2F{e_2}{O_3}.n{H_2}O \\
  \left( {metal} \right)\,\,\,\,\,\,\,\,\,\,\,\,\left( {oxygen} \right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {water} \right) \\
 $
Hence, we can conclude from the above gathered information that ;
The correct option is: (C) Water, oxygen, and metal.

Note:
The method of applying a protective zinc coating to steel or iron to avoid rusting is known as galvanization or galvanising. The most popular method is hot-dip galvanising, which involves immersing the pieces in molten zinc.