Milk turning sour is a chemical change.
A chemical change is a process that involves a chemical reaction resulting in the formation of new substances with different properties from the original substances. In the case of milk turning sour, lactic acid bacteria in the milk convert lactose (a sugar present in milk) into lactic acid through a process called fermentation. This results in a change in the composition of milk, which now has a different taste, texture, and odor than fresh milk.
On the other hand, dissolving sugar in water, melting of ice, and crystallization are physical changes that do not involve a change in the chemical composition of the substances.
Therefore, the correct answer is "Milk turning sour."