Hihi!
The answer would be D. milk turning sour! What happens when milk goes sour is it ferment lactose at room temperature and which turns it into lactic acid. More acid means more hydrogen ions, a lower pH and a sour taste. Lower pH change the structure of proteins like casein, which gives milk its whiteness, so it curdles into clumps called curds! It's pretty gross!
I hope I helped!
-Jailbaitasmr
