What is the primary purpose of vitamin D in humans?​ ​

A) ​to develop acute eyesight
B) ​to maintain the balance of calcium in the body
C) ​to prevent the development of respiratory illnesses
D) ​to enhance the production of collagen for hair

The primary purpose of vitamin D in humans is to maintain the balance of calcium in the body by promoting its absorption in the gut and reducing its excretion by the kidneys. This helps in building and maintaining strong bones, teeth, and muscles. Vitamin D also plays a role in supporting the immune system and reducing inflammation. It is not primarily involved in developing eyesight, preventing respiratory illnesses, or enhancing collagen production for hair.