An 8-week old boy presents to his pediatrician for his 2 month well baby examination. He was the product of a full-term vaginal delivery to a G2P2 34-year old mother following an uneventful pregnancy. His mother took prenatal vitamins throughout pregnancy but no other supplements or medications. His Apgar scores were 8 at 1 minute and 9 at 5 minutes. His parents have refused the vitamin K injection and hepatitis B vaccine. He is exclusively breastfed and his mother is still taking prenatal vitamins. Of the following, choose the best choice to be supplemented in his diet: A. Vitamin K B. Fluoride C. Vitamin D D. Calcium.