Concept:Vitamin D is the key nutrient that enables the body to absorb calcium from the digestive tract, which is essential for building and maintaining strong bones and teeth.
Explanation:1. Calcium absorption in the small intestine depends on the active form of vitamin D (calcitriol). Without sufficient vitamin D, only a small fraction of dietary calcium is absorbed.
2. Vitamin D also helps regulate the levels of calcium and phosphate in the blood, ensuring that bones remain mineralised and healthy.
3. The body can obtain vitamin D from sunlight exposure (UVB rays trigger synthesis in the skin), from foods such as fatty fish, egg yolks, and fortified dairy products, and from supplements.
4. A deficiency of vitamin D leads to poor calcium absorption, which can cause soft, weak bones — rickets in children and osteomalacia in adults. Long‑term deficiency increases the risk of osteoporosis and fractures.
5. While calcium itself is a mineral stored in bones and obtained from dairy, leafy greens, and nuts, it cannot be used effectively without adequate vitamin D.
Answer:Vitamin D (Option D) is the correct answer.