C.

After a wine’s initial fermentation (during which, yeasts turn grape sugar into alcohol), a winemaker may choose to let the grapes undergo a process called malolactic fermentation. This process softens the wine by converting malic acid to lactic acid, and creates a byproduct called diacetyl. This compound is what gives butter its characteristic flavor, so when it is present in wine, diacetyl can have the same effect.