Which of the following styles was created by nineteenth-century west coast brewers using warm fermentation with lager yeast?

The style known as California Common was indeed crafted by nineteenth-century west coast brewers who utilized a unique technique involving warm fermentation with lager yeast. This method allowed them to create a beer that exhibited the clean, crisp characteristics typical of lagers, while also showcasing some of the fruity and aromatic notes more commonly associated with ales, especially due to the warmer fermentation temperatures.

California Common is particularly notable because it was developed to adapt to the local brewing conditions found on the West Coast, where lager yeast is typically affected by warmer ambient temperatures. This led to a distinctive style that is often characterized as having a moderate hop flavor and a well-balanced malt backbone, reflecting the ingredients available at the time and the innovative brewing techniques being pursued.

The other styles mentioned, such as American Lager, Porter, and Stout, do not share the same fermentation method or historical context associated with California Common. American Lager is traditionally fermented at lower temperatures using lager yeast, while Porter and Stout are styles that arose much earlier and are primarily associated with ales and darker, richer malt profiles. Therefore, California Common stands out because it represents a regional adaptation and evolution of lager brewing in the face of specific environmental challenges.