Which of the following is a sign that a product might not be gluten-free?

Oats can sometimes be a sign that a product might not be gluten-free due to the risk of cross-contamination during processing. While oats themselves do not contain gluten, they are often processed in facilities that also handle gluten-containing grains such as wheat, barley, and rye. This cross-contamination can lead to the presence of gluten in oat products, making it unsafe for individuals with celiac disease or gluten intolerance.

In contrast, corn, rice, and quinoa are naturally gluten-free grains and are generally considered safe for those who need to avoid gluten, as long as they are not contaminated during processing. Thus, the presence of oats is a key indicator that a product may not be gluten-free, mainly due to the potential for gluten contamination rather than the oats themselves containing gluten.