**Core Concept**
Vitamins are organic compounds required by the body for various metabolic processes. Hormonal vitamins are a subset of vitamins that play a crucial role in hormone regulation and synthesis. These vitamins act as coenzymes or cofactors for enzymes involved in hormone production.

**Why the Correct Answer is Right**
Vitamin D is the only vitamin that can be classified as a hormonal vitamin. It is synthesized in the skin upon exposure to sunlight and plays a critical role in calcium homeostasis and bone metabolism. Vitamin D acts as a steroid hormone and regulates the expression of genes involved in calcium absorption and bone mineralization. It also has immunomodulatory effects and is involved in the regulation of cell growth and differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin A is an essential vitamin for vision, immune function, and reproduction, but it is not classified as a hormonal vitamin.
**Option B:** Vitamin B complex is a group of vitamins that play a crucial role in energy metabolism, but none of them are hormonal vitamins.
**Option C:** Vitamin E is an antioxidant vitamin that protects cells from oxidative damage, but it is not a hormonal vitamin.

**Clinical Pearl / High-Yield Fact**
Vitamin D deficiency is a common condition that can lead to osteoporosis, osteopenia, and increased 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: D. Vitamin D.**