The correct formula of aluminium oxide is

1. AlO₃
2. AlO₂
3. Al₂O₃
4. Al₃O₂

Al₂O₃

Reason — The formula of aluminium oxide can be determined as below:

$\text{Al}^{3+} \space \text{O}^{2-} \\[1em] \overset{\phantom{3}{3}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{2}{2}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{3}{3}}{\text{O}} \\[1em] \text{Al}$2\text{O}3