Option 4, Na2O, is the correct answer. Na2O is known as sodium oxide, and it is an oxide compound that dissolves in water. When Na2O is added to water, it reacts to form sodium hydroxide (NaOH), a strong base. This reaction is exothermic, meaning it releases heat. Sodium hydroxide is a highly soluble compound in water, which is why Na2O is able to dissolve in water.

On the other hand, options 1, 2, and 3, CuO, Al2O3, and Fe2O3 respectively, do not dissolve in water. CuO is copper oxide, Al2O3 is aluminum oxide, and Fe2O3 is iron(III) oxide. These compounds are all classified as metal oxides, which are generally not soluble in water. Metal oxides tend to form basic solutions when they react with water, but they do not dissolve completely in water. Instead, they may form hydroxide ions (OH-) that can react with water to form hydroxide compounds, such as copper hydroxide (Cu(OH)2), aluminum hydroxide (Al(OH)3), and iron(III) hydro