**Core Concept**
The gene for the folic acid transporter is a crucial component in the metabolism of folate, an essential nutrient for DNA synthesis and repair. The folic acid transporter is responsible for the uptake of folate into cells, which is a critical step in maintaining adequate folate levels in the body.

**Why the Correct Answer is Right**
The gene for the folic acid transporter, also known as SLC19A1, is located on chromosome 21. This gene encodes a protein that functions as a folate transporter, allowing cells to take up folate from the extracellular environment. The folate transporter is essential for the proper functioning of folate-dependent enzymes, such as methionine synthase, which is involved in the conversion of homocysteine to methionine. Mutations in the SLC19A1 gene have been associated with impaired folate uptake and increased risk of certain diseases, including neural tube defects.

**Why Each Wrong Option is Incorrect**
**Option A:** Chromosome 5 is not associated with the gene for the folic acid transporter. Chromosome 5 is involved in various other genetic disorders, such as cystic fibrosis, but not folate metabolism.
**Option B:** Chromosome 15 is not linked to the gene for the folic acid transporter. Chromosome 15 is involved in genetic disorders such as Prader-Willi syndrome and Angelman syndrome, but not folate metabolism.
**Option D:** The gene for the folic acid transporter is not located on the X chromosome. While the X chromosome is involved in various genetic disorders, the SLC19A1 gene is autosomal, meaning it is located on one of the non-sex chromosomes.

**Clinical Pearl / High-Yield Fact**
The folate transporter is also involved in the transport of antifolate drugs, such as methotrexate, which are used in the treatment of certain cancers and autoimmune diseases. Understanding the role of the folate transporter in folate metabolism is crucial for the development of effective therapies for folate-related disorders.

**✓ Correct Answer: C. 21**