**Core Concept**
Vitamin D is a fat-soluble vitamin that plays a crucial role in bone health, immune function, and cell growth. It is synthesized in the skin upon exposure to sunlight and can also be obtained through dietary sources.

**Why the Correct Answer is Right**
Vitamin D is synthesized in the skin from 7-dehydrocholesterol by the enzyme cholecalciferol (Vitamin D3). This process occurs when 7-dehydrocholesterol is exposed to UVB rays from sunlight. The resulting Vitamin D3 is then transported to the liver and kidneys, where it is converted into its active form, calcitriol. Calcitriol then acts on the intestines to increase calcium absorption, which is essential for maintaining bone health.

**Why Each Wrong Option is Incorrect**
**Option A:** Some sources may claim that Vitamin D is maximum in fish oil, but this is incorrect as fish oil is a source of Vitamin D, not the maximum source.
**Option B:** Vitamin D is not maximum in cod liver oil, although it is a rich source of Vitamin D.
**Option C:** Vitamin D is not maximum in fortified milk, although it is a common source of Vitamin D.

**Clinical Pearl / High-Yield Fact**
Vitamin D deficiency is a common problem worldwide, especially in individuals with limited sun exposure, such as those living in northern latitudes or with darker skin.

**Correct Answer: B. Cod liver oil.