Step 1: Understand what rust is.
Rust is the reddish-brown layer that forms on iron when it is left in damp air. It is a kind of corrosion that slowly eats away iron objects.
Step 2: See how rust forms.
Iron reacts with both oxygen and water (moisture) from the air. In short, iron + oxygen + water gives rust.
Step 3: Note that water is part of rust.
Rust is not just dry iron oxide. It traps water molecules inside it, so it is called a hydrated oxide. The formula is $Fe_2O_3 \cdot xH_2O$.
Step 4: Identify the iron state.
The iron in rust is in the ferric form, that is $Fe^{3+}$. So the oxide is ferric oxide, $Fe_2O_3$, not ferrous oxide.
Step 5: Put it together.
Combining the ferric oxide with the trapped water, rust is hydrated ferric oxide.
Step 6: Check the options and conclude.
It is not ferrous (that is $Fe^{2+}$), and not pure dry ferric oxide because water is present. So rust is hydrated ferric oxide.
\[ \boxed{\text{Hydrated ferric oxide}} \]