Which one is the most acidic among these?

The correct option is Al₂O₃.

To understand why Al₂O₃ (aluminium oxide) is the most acidic among the given options, we need to analyze the periodic trends of metallic character and oxide behavior across a period.

The given oxides consist of elements from Period 3 (Na, Mg, Al) and Period 4 (Ca) of the periodic table:
- Na₂O (Sodium oxide)
- MgO (Magnesium oxide)
- Al₂O₃ (Aluminium oxide)
- CaO (Calcium oxide)

Generally, metallic oxides are basic in nature, while non-metallic oxides are acidic. Across a period from left to right (e.g., from Na to Mg to Al in Period 3), the metallic character decreases and the electronegativity increases. As a result, the oxides transition from strongly basic to amphoteric, and eventually to acidic (for non-metals like silicon, phosphorus, and sulfur).

Let's look at the nature of each oxide:
1. Na₂O: Sodium is an alkali metal (Group 1). Its oxide is strongly basic and dissolves in water to form a strong base, NaOH.
2. CaO: Calcium is an alkaline earth metal (Group 2, Period 4). It is highly metallic, making CaO a strongly basic oxide that reacts with water to form Ca(OH)₂.
3. MgO: Magnesium is also an alkaline earth metal (Group 2, Period 3). MgO is basic, though less basic than Na₂O and CaO.
4. Al₂O₃: Aluminium is a metalloid-like metal in Group 13. Its oxide, Al₂O₃, is amphoteric, meaning it exhibits both basic and acidic properties. It can react with both acids and strong bases.

Because Al₂O₃ is amphoteric, it exhibits significant acidic character compared to the other three oxides (Na₂O, CaO, and MgO), which are purely basic. Therefore, Al₂O₃ is the most acidic (or least basic) oxide among the choices.