What is considered the main food source of vitamin D?

Vitamin D is essential for maintaining healthy bones and supporting immune function, and its primary food sources are fatty fish and fortified dairy products. Fatty fish, such as salmon and mackerel, contain significant amounts of vitamin D in a form that is readily usable by the body. Additionally, many dairy products, such as milk and yogurt, are fortified with vitamin D to help meet the daily requirements of this important nutrient.

This combination of natural and fortified food sources makes option C the most accurate choice. While leafy green vegetables, whole grains, and nuts and seeds offer other valuable nutrients, they are not significant sources of vitamin D.