The correct answer is A. Niacin.
Pellagra is a disease caused by a deficiency of niacin, a B vitamin. It is characterized by diarrhea, dermatitis, and dementia. Niacin is essential for the production of energy and the synthesis of DNA and RNA. It is also involved in the function of the nervous system and the immune system.
Riboflavin, thiamine, and pyridoxine are also B vitamins, but they are not associated with pellagra. Riboflavin is involved in the metabolism of carbohydrates, fats, and proteins. Thiamine is involved in the metabolism of carbohydrates and the function of the nervous system. Pyridoxine is involved in the metabolism of proteins and the function of the nervous system.
Pellagra is a preventable disease. It can be treated with niacin supplements or a diet that is high in niacin-rich foods, such as meat, poultry, fish, eggs, dairy products, legumes, nuts, and seeds.