The single most important ingredient in a good beer is high-quality a. yeast. b. water. c. malt. d. hops.

D. hops

A beer is a beverage which consists of  90% water, barley, hops and yeast. Yeast is a type of fungus that is used to conduct natural fermentation of maltose sugar in the barley grain. Hops is the ingredient which is used to enhance the quality and flavor to the beer drink. It is added to balance the extreme sweetness of the maltose sugars in the beer drink and provides it a bitter taste.  

it's all four options.