Which of the following is not an ingredient used in sake production?

Barley is indeed not an ingredient used in sake production. Sake is primarily made from rice, water, yeast, and koji mold. The staple ingredient is polished rice, which is fermented using these components.

Water plays a crucial role as it affects the taste of the sake, and the yeast is responsible for converting the sugars in the rice into alcohol during fermentation. Koji mold is used to convert the starches in rice into fermentable sugars, which are essential in the brewing process. Barley, while a common ingredient in beer, does not have a role in the production of sake, confirming that it is the correct answer in this context.