Which of the following grains is NOT allowed in a gluten-free diet?

Barley is the grain that is not allowed in a gluten-free diet. This is primarily due to the presence of gluten, which is a protein found in certain grains, including barley. Gluten can trigger adverse reactions in individuals with celiac disease or gluten intolerance. Therefore, any food products containing barley would not be suitable for those adhering to a gluten-free regimen.

In contrast, corn, rice, and quinoa are all considered gluten-free grains. Corn is a common staple in gluten-free diets, while rice is often recommended as a safe alternative. Quinoa, although not a traditional grain, is also gluten-free and packed with nutrients, making it a popular choice. Thus, the inclusion of barley in diets intended to be gluten-free is what makes it inappropriate for those needing to avoid gluten.