**Core Concept**
The folate carrier protein, also known as the reduced folate carrier (RFC), plays a crucial role in the transport of folate and its derivatives across cell membranes. This protein is essential for the proper absorption and utilization of folate, a vitamin that is critical for DNA synthesis and repair.

**Why the Correct Answer is Right**
The gene encoding the reduced folate carrier protein, SLC19A1, is located on chromosome 21. This gene is responsible for encoding a protein that facilitates the transport of folate into cells, where it can be used for various cellular processes, including DNA synthesis and repair. The RFC protein is a carrier-mediated transport protein that uses a process called facilitated diffusion to transport folate across cell membranes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the gene for the reduced folate carrier protein is not located on chromosome 11. The correct chromosome is chromosome 21.
**Option B:** This option is incorrect because the gene for the reduced folate carrier protein is not located on chromosome 18. The correct chromosome is chromosome 21.
**Option C:** This option is incorrect because the gene for the reduced folate carrier protein is not located on chromosome 5. The correct chromosome is chromosome 21.

**Clinical Pearl / High-Yield Fact**
It's worth noting that mutations in the SLC19A1 gene, which encodes the reduced folate carrier protein, have been associated with hereditary folate malabsorption, a rare genetic disorder that affects the body's ability to absorb folate.

**Correct Answer:** D. Chromosome 21.