Which ingredient is essential for contributing aroma to beer?

D is the correct choice because hops are known for their significant contribution to the aroma of beer. When hops are added during the brewing process, particularly towards the end of boiling or during fermentation in a method called dry hopping, they impart a wide range of aromatic qualities to the beer. These aromas can vary widely, including floral, citrus, pine, herbal, and spicy notes, depending on the hop variety used. This characteristic is one of the primary reasons hops are a crucial ingredient in brewing.

While yeast does produce some aroma, primarily through fermentation byproducts, its main role is in the fermentation process rather than specifically contributing to the aroma like hops. Although malt can add some sweetness and bread-like aromas, its contribution is generally more about flavor and body. Water, while essential for the brewing process, does not contribute aroma directly. Therefore, the unique aromatic qualities of hops make them the indispensable ingredient for contributing aroma in beer.