Name the four main ingredients of beer.

The main ingredients of beer are water, malted barley, hops, and yeast.

Water is the primary component of beer, making up the majority of its content and affecting the flavor based on its mineral composition. Malted barley serves as the source of fermentable sugars, which are crucial for alcohol production during fermentation. Hops provide bitterness, aroma, and flavor, helping to balance the sweetness of the malt and contributing to the beer's overall character. Yeast is essential for the fermentation process, as it consumes the sugars from the malt and produces alcohol and carbon dioxide.

The other options include ingredients that are not fundamental to the basic composition of beer. For instance, sugar can be added in various forms, but it is not one of the four main ingredients. Similarly, while corn can be used in brewing, particularly in certain styles of beer, it is not a primary ingredient like malted barley. Thus, the correct answer accurately encompasses the traditional and essential ingredients involved in brewing beer.