Which metal ion produces the only white precipitate that dissolves in excess sodium hydroxide solution?

The correct answer is aluminium hydroxide. When aluminium ions react with sodium hydroxide, they initially form a white precipitate of aluminium hydroxide. This compound is unique among the options presented because it is amphoteric, meaning it can react with both acids and bases.

In the case of excess sodium hydroxide, aluminium hydroxide will dissolve to form a soluble aluminate ion, which is not the case with the other precipitates listed.

Magnesium hydroxide, copper hydroxide, and calcium hydroxide all produce white precipitates as well, but they do not dissolve in excess sodium hydroxide. Magnesium hydroxide and calcium hydroxide are generally insoluble in excess base, maintaining their precipitate form, while copper hydroxide, although it also appears as a precipitate, does not show the same behavior under alkaline conditions.

Therefore, aluminium hydroxide's ability to dissolve when excess sodium hydroxide is added makes it the only metal ion that produces a white precipitate that is soluble in this scenario.