**Core Concept**
The question tests the understanding of **vitamin** roles in **metabolic pathways**, specifically the transfer of **1-carbon units**. This process is crucial in the synthesis of **nucleic acids** and **amino acids**. **Folic acid** plays a key role in this biochemical reaction.

**Why the Correct Answer is Right**
**Folic acid** or **Vitamin B9** is essential for the transfer of 1-carbon units in various metabolic reactions, including the synthesis of **purines**, **pyrimidines**, and **amino acids** like **methionine**. It acts through its coenzyme form, **tetrahydrofolate (THF)**, which donates or accepts 1-carbon units at different oxidation levels. This process is vital for **DNA synthesis** and repair.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while other vitamins are crucial for various metabolic functions, they are not directly involved in the transfer of 1-carbon units.
**Option B:** Incorrect as it does not directly participate in 1-carbon metabolism.
**Option C:** Incorrect because, although important, it is not the primary vitamin for this specific biochemical process.

**Clinical Pearl / High-Yield Fact**
Remember, **folic acid deficiency** can lead to **megaloblastic anemia**, emphasizing its critical role in **DNA synthesis** and, by extension, **cell division**. This correlation is a key point for clinical diagnosis and treatment.

**Correct Answer:** D. Folic acid