Option (C) is correct because oxygen combines with iron found in the rocks to form iron oxides. Rust is another name for iron oxide. When iron or an alloy that contains iron is exposed to oxygen and moisture for a long period of time, a new compound is formed called iron oxide.

Option (A) is not correct because carbon dioxide combines with iron to form iron carbonate. It occurs naturally as the mineral siderite.

Option (B) is not correct because nitrogen combines with iron to form iron nitrites.

Option (D) is not correct because sulphur combines with iron to form iron sulphate. It is a salt and soluble in water.