During the rusting of an iron nail, iron and oxygen combine to form iron oxide. Rusting is an electrochemical process that typically occurs in the presence of moisture and can be represented by the equation:
\[ 4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3 \]
The iron (Fe) reacts with oxygen (O₂) and water (H₂O), eventually leading to the formation of hydrated iron(III) oxide, commonly known as rust. Therefore, the correct response is:
Iron and oxygen combine to form iron oxide.