The correct answer is option 4: Sun. Sunlight is the primary source of vitamin D for human beings. When our skin is exposed to sunlight, it synthesizes a form of vitamin D called cholecalciferol. This inactive form of vitamin D then undergoes a series of metabolic reactions in the liver and kidneys to be converted into its active form, known as calcitriol.

Citrus fruits, option 1, are not a significant source of vitamin D. While they do contain other vitamins and minerals, such as vitamin C, they do not naturally provide vitamin D.

Green vegetables, option 2, are generally not a source of vitamin D either. Although they offer a variety of other essential nutrients, such as vitamins A and K, they typically do not contain vitamin D unless they have been fortified with it.

Yeast, option 3, is not a primary source of vitamin D. While yeast can be fortified with vitamin D, it is not a natural source and is not commonly consumed in significant amounts to meet vitamin D needs.

It is important to note that while sunlight is the primary source of vitamin D, it is also possible to obtain vitamin D through certain foods, such as fatty fish, eggs, and fortified dairy products. However,