Final answer:
Humans obtain vitamin D through both diet and exposure to sunlight. The skin synthesizes vitamin D3 when exposed to UV light; additionally, diet can provide vitamin D, especially through fatty fish and fortified foods. option d
Step-by-step explanation:
Humans obtain vitamin D from both diet and sunlight. When exposed to UV radiation, the epidermal layer of human skin synthesizes a form of vitamin D3 called cholecalciferol. This is then converted to calcidiol in the liver and to its active form, calcitriol, in the kidneys. Vitamin D is crucial for the absorption of calcium and phosphorous necessary for healthy bones. Lack of vitamin D can lead to conditions such as rickets in children and osteomalacia in adults. Dietary sources include fatty fish like salmon and tuna, fortified milk, or cereal, but many foods do not naturally contain vitamin D. Therefore, both diet and sunlight exposure are important for maintaining adequate levels of vitamin D in the body.