The oxides of the elements across period 3, from sodium to sulphur, change from

Explanation

Sodium and magnesium oxides are basic, aluminium oxide is amphoteric and reacts with both acids and alkalis, and the oxides of phosphorus and sulphur are acidic, dissolving to give acids. The change follows the shift from metallic to non metallic character across the period. Identifying aluminium oxide as the amphoteric turning point is the key fact.