Final answer:
The FDA defines 'gluten-free' as products containing less than 20 ppm of gluten, which is significant for those with celiac disease.
Step-by-step explanation:
The FDA definition of gluten-free is A. Products containing <20 ppm of gluten. This means that in order for a food to be labeled as 'gluten-free', it must contain less than 20 parts per million (ppm) of gluten. This is a critical standard for people with celiac disease, who must avoid gluten entirely to prevent an immune response that can damage the small intestine's lining and lead to malnutrition and other serious health issues.