**Core Concept**
Vitamins are essential nutrients that play critical roles in various bodily functions, including energy production, immune function, and growth and development. Hormonal vitamins are a subset of vitamins that have a hormonal effect on the body, influencing gene expression and cellular metabolism.

**Why the Correct Answer is Right**
Vitamin D is the primary example of a hormonal vitamin. It is produced in the skin upon exposure to sunlight and functions as a steroid hormone, influencing calcium absorption and bone metabolism. Vitamin D exerts its effects through binding to the vitamin D receptor (VDR) in target cells, which is a nuclear receptor that regulates gene expression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not specify a vitamin that has a hormonal effect on the body. While many vitamins have essential roles in metabolism, not all vitamins have hormonal functions.
**Option B:** This option is also incorrect because it does not mention a vitamin that is known to have hormonal effects. B vitamins are crucial for energy metabolism, but they do not have the same level of hormonal activity as vitamin D.

**Clinical Pearl / High-Yield Fact**
Vitamin D deficiency is a common issue worldwide, particularly in individuals with limited sun exposure, dark skin, or those with gastrointestinal disorders. It is essential to recognize vitamin D deficiency, as it can lead to osteomalacia, osteoporosis, and increased risk of fractures.

**Correct Answer: D. Vitamin D.**