A buttery flavor in wine is most often the result of:

C.

While many people think that buttery flavors in wine are derived from new oak, they are actually the result of diacetyl, which is a by-product of malolactic fermentation. (For its part, malolactic fermentation is the process by which crisp malic acid in wine is converted to softer lactic acid). Chardonnays that have undergone malolactic fermentation often have considerable diacetyl and therefore a pronounced buttery taste. Bâttonage means lees stirring in French, from bâton French for stick.