**Core Concept**
Vitamin D is a unique nutrient that functions as both a vitamin and a hormone in the body. It plays a crucial role in maintaining calcium and phosphate homeostasis, which is essential for bone health.

**Why the Correct Answer is Right**
Vitamin D is synthesized in the skin upon exposure to sunlight and also obtained from dietary sources. After absorption, it undergoes hydroxylation in the liver and kidneys to form the active hormone, calcitriol (1,25-dihydroxyvitamin D). Calcitriol then acts on the intestine, kidneys, and bone to regulate calcium and phosphate levels. This hormone-like activity of vitamin D is mediated through its binding to the vitamin D receptor (VDR) in target cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin A is essential for vision, immune function, and skin health, but it does not have a hormone-like function.
**Option B:** Vitamin C is crucial for collagen synthesis, iron absorption, and antioxidant defenses, but it does not have a hormone-like activity.
**Option D:** Vitamin E is a powerful antioxidant that protects cells from damage, but it does not have a hormone-like function.

**Clinical Pearl / High-Yield Fact**
Remember that vitamin D deficiency can lead to impaired bone mineralization, increased risk of osteoporosis, and an elevated risk of fractures. It is essential to maintain adequate vitamin D levels, especially in individuals with limited sun exposure or those with malabsorption syndromes.

**✓ Correct Answer: C. D Vitamin D**