**Core Concept**
Folic acid, a B vitamin, is essential for various bodily functions, including DNA synthesis and repair, cell division, and growth. The active form of folic acid is necessary for the proper functioning of these processes, particularly in the synthesis of nucleic acids.

**Why the Correct Answer is Right**
The active form of folic acid is tetrahydrofolate (THF), which is formed through the reduction of folic acid in the liver by the enzyme dihydrofolate reductase (DHFR). THF serves as a cofactor for several enzymes involved in one-carbon metabolism, including thymidylate synthase, which is crucial for DNA synthesis. This process is essential for the rapid cell division seen in rapidly proliferating cells, such as cancer cells or embryonic cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Folic acid is incorrect because it is the inactive form of the vitamin, which must be converted to its active form, THF, before it can be used by the body.
**Option B:** 5-methyltetrahydrofolate (5-MTHF) is incorrect because, although it is a derivative of THF, it is not the primary active form of folic acid. 5-MTHF is involved in the conversion of homocysteine to methionine but is not the initial active form of folic acid.
**Option C:** Dihydrofolate (DHF) is incorrect because it is an intermediate in the conversion of folic acid to THF but is not the active form of the vitamin.

**Clinical Pearl / High-Yield Fact**
Folic acid supplementation is essential during pregnancy to prevent neural tube defects, such as spina bifida, and is also crucial for patients taking antifolate medications, such as methotrexate, to prevent adverse effects on rapidly proliferating cells.

**Correct Answer: B. 5-methyltetrahydrofolate (5-MTHF)**