When milk turns sour, in which direction does its pH change?

Correct answer: pH decreases

When milk turns sour, microorganisms convert lactose into lactic acid through fermentation. The increased acid concentration raises hydrogen-ion concentration, so the pH decreases. Therefore option A is correct. A rise in pH would indicate a movement toward basicity, not sourness. The pH does not become fixed at 14, and it cannot remain unchanged when an acidic product is being formed.