**Core Concept**
Vitamin D is an essential nutrient that plays a crucial role in maintaining bone health by regulating calcium levels. It is obtained from two primary sources: cutaneous synthesis through sunlight exposure and dietary intake.

**Why the Correct Answer is Right**
Vitamin D is primarily synthesized in the skin upon exposure to ultraviolet B (UVB) rays from the sun. This process occurs in the presence of 7-dehydrocholesterol, a cholesterol derivative, which is converted to pre-vitamin D3. Subsequently, pre-vitamin D3 is transported to the liver and kidneys, where it undergoes further hydroxylation to form the active form of Vitamin D, calcitriol (1,25-dihydroxyvitamin D). The dietary intake of Vitamin D is minimal, and it is mainly obtained through supplements or fortified foods.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be incorrect, but Vitamin D can be found in some plant-based sources, albeit in lower quantities. However, the primary concern here is the insufficiency of Vitamin D in these sources.
**Option B:** This option might be a distractor, but Vitamin D is also available in animal-based products like fatty fish (salmon, mackerel, and sardines). However, the question is about sufficient quantities.
**Option C:** This option is likely the correct answer because Vitamin D is not found in sufficient quantities in plant-based products like vegetables, fruits, and grains. These sources often require fortification to provide adequate amounts of Vitamin D.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Vitamin D deficiency is a common issue worldwide, particularly in individuals with limited sun exposure, dark skin, or those who follow a strict vegetarian or vegan diet. This deficiency can lead to various health issues, including rickets in children and osteomalacia in adults.

**Correct Answer:** C. Plant-based products.