The correct answer is option 4: vitamin D. Vitamin D is unique because it can be synthesized by our own bodies when our skin is exposed to sunlight. When UV rays from the sun hit our skin, a compound called 7-dehydrocholesterol is converted into a form of vitamin D called cholecalciferol or vitamin D3. This inactive form is then transported to the liver and kidneys, where it is converted into its active form, calcitriol. However, it is important to note that our bodies can only manufacture vitamin D when exposed to sunlight, so getting enough sun exposure or consuming foods rich in vitamin D (such as fatty fish, egg yolks, and fortified dairy products) is crucial for maintaining adequate levels of this essential vitamin.