Which vitamin is produced when humans are exposed to sunlight?

When humans are exposed to sunlight, specifically ultraviolet (UV) rays, the skin synthesizes Vitamin D through a process that begins with the conversion of a cholesterol derivative in the skin. The UVB rays convert 7-dehydrocholesterol to previtamin D3, which then becomes vitamin D3 (cholecalciferol) and is ultimately converted into the active form of vitamin D in the liver and kidneys.

Vitamin D plays a crucial role in calcium absorption and bone health, contributing to the maintenance of strong bones and teeth. It also supports immune system function and has various other health benefits. This unique ability of our skin to produce Vitamin D through sunlight exposure distinguishes it from vitamins like A, B12, and C, which must be obtained through diet or supplements.

Thus, the correct answer is Vitamin D, as it is the only vitamin that humans can synthesize in response to sunlight exposure.