**Core Concept**
Folic acid supplementation is essential during pregnancy to prevent neural tube defects (NTDs) in the developing fetus. NTDs arise from the incomplete closure of the neural tube, which gives rise to the brain and spinal cord. Folic acid plays a crucial role in DNA synthesis and repair, ensuring proper neural tube formation.

**Why the Correct Answer is Right**
Folic acid supplementation, particularly in the first trimester, significantly reduces the risk of NTDs. This is because folic acid is a crucial cofactor for the enzyme methionine synthase, which is involved in the metabolism of homocysteine. Elevated homocysteine levels have been associated with an increased risk of NTDs. Folic acid supplementation helps maintain healthy homocysteine levels, thereby reducing the risk of NTDs. The recommended daily dose of folic acid for pregnant women is 400-800 μg.

**Why Each Wrong Option is Incorrect**
**Option A:** Iron is essential for pregnant women to prevent anemia, but it does not have a direct role in preventing NTDs.
**Option B:** Vitamin B12 is crucial for the formation of red blood cells and the maintenance of the nervous system, but it is not directly involved in preventing NTDs.
**Option D:** Zinc is an essential mineral for various bodily functions, including immune function and wound healing, but it does not have a proven role in preventing NTDs.

**Clinical Pearl / High-Yield Fact**
Pregnant women should start folic acid supplementation at least 1 month before conception and continue it throughout the first trimester to effectively prevent NTDs. A diet rich in folate-containing foods, such as leafy greens and legumes, can also contribute to adequate folic acid levels.

**Correct Answer:** ✓ Correct Answer: C. Folic acid