**Core Concept**
Folic acid plays a crucial role in fetal development during early pregnancy, particularly in the prevention of neural tube defects (NTDs). A deficiency in one of the haemophilic factors, which is essential for folic acid metabolism, can increase the risk of NTDs.

**Why the Correct Answer is Right**
The correct haemophilic factor is **Factor C (also known as cystathionine beta-synthase)**, which is involved in the metabolism of homocysteine to methionine. Methionine is then converted to S-adenosylmethionine (SAMe), a key methyl donor for the conversion of folic acid to its active form, 5-methyltetrahydrofolate (5-MTHF). The latter is crucial for the synthesis of nucleic acids and the prevention of NTDs. A deficiency in Factor C leads to elevated homocysteine levels, which can impair folic acid metabolism and increase the risk of NTDs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Factor I (also known as plasminogen activator inhibitor-1) is involved in the regulation of fibrinolysis, not folic acid metabolism.

**Option B:** This option is incorrect because Factor II (also known as prothrombin) is a vitamin K-dependent clotting factor, not directly involved in folic acid metabolism.

**Option C:** This option is incorrect because Factor V (also known as proaccelerin) is involved in the coagulation cascade, not folic acid metabolism.

**Clinical Pearl / High-Yield Fact**
A deficiency in Factor C (cystathionine beta-synthase) can lead to homocystinuria, a condition characterized by elevated homocysteine levels and an increased risk of NTDs, cardiovascular disease, and other complications. This emphasizes the importance of maintaining optimal homocysteine levels through adequate folic acid supplementation during pregnancy.

**Correct Answer:** C.