**Core Concept**
Exclusively breastfed babies have a unique nutritional profile that requires careful consideration, especially in the first few months of life. Vitamin B12 plays a crucial role in the development of the central nervous system, and its deficiency can lead to severe consequences.

**Why the Correct Answer is Right**
Vitamin B12 is essential for the synthesis of myelin, the fatty substance that surrounds and protects nerve fibers. Breast milk contains very little vitamin B12, and exclusively breastfed babies rely on their mother's stores, which are depleted over time. If a mother's vitamin B12 levels are low, her breast milk will not contain sufficient amounts to meet the baby's needs, leading to a deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Folate - Folate is another essential vitamin for growth and development, but it is not specifically deficient in exclusively breastfed babies.
**Option B:** Vitamin D - While vitamin D is crucial for bone health, exclusively breastfed babies are not at a higher risk of deficiency if their mothers have adequate vitamin D levels.
**Option C:** Iron - Iron is essential for healthy red blood cells, but breast milk contains sufficient iron for exclusively breastfed babies, especially if their mothers have adequate iron stores.

**Clinical Pearl / High-Yield Fact**
Exclusively breastfed babies are at risk of vitamin B12 deficiency if their mothers have a vegetarian or vegan diet, as animal products are the primary source of vitamin B12.

**Correct Answer:** C. Vitamin B12.