The correct answer is option 4: Calcium oxide.

When sodium hydroxide reacts with metal oxides, it typically forms a corresponding metal hydroxide and water. However, calcium oxide (CaO) is an exception to this general trend. Calcium oxide does not usually react with sodium hydroxide.

Option 1: Copper oxide (CuO) can react with sodium hydroxide to form copper hydroxide (Cu(OH)2).

Option 2: Aluminium oxide (Al2O3) can react with sodium hydroxide to form sodium aluminate (NaAlO2).

Option 3: Potassium oxide (K2O) can react with sodium hydroxide to form potassium hydroxide (KOH).

Therefore, option 4 is the correct answer because calcium oxide does not usually react with sodium hydroxide, unlike the other metal oxides mentioned.