storing milk by putting it into the freezer milk turning sour when left in a warm place mixing milk into a glass with chocolate powder allowing milk to evaporate, leaving behind powdered milk
I gave my students this multiple choice question: Which of these is an example of a chemical reaction?

Only one answer is correct among these 4 choices:
- storing milk by putting it into the freezer
- milk turning sour when left in a warm place
- mixing milk into a glass with chocolate powder
- allowing milk to evaporate, leaving behind powdered milk

Could you tell me which one is correct?

milk turning sour when left in a warm place