When iron is exposed to moist air, it gradually rusts. This is due to the formation of

Correct answer: hydrated iron(III) oxide.

Explanation: Rust is the reddish-brown compound formed when iron reacts with oxygen and water in moist air. Its main component is hydrated iron(III) oxide, commonly written as Fe₂O₃·xH₂O.

• Anhydrous iron(III) oxide (Fe₂O₃) — lacks water; that’s not rust.
• Iron(II) oxides — Fe_O/hydrated Fe(II) may form transiently, but rust is predominantly Fe(III) and hydrated.