**Core Concept**
The ABO blood group system is controlled by a single gene with three alleles, determining an individual's blood type. This gene is located on a specific **chromosome** and encodes for **glycosyltransferases**, which are enzymes responsible for the synthesis of the ABO blood group antigens. The **ABO gene** is crucial in transfusion medicine.

**Why the Correct Answer is Right**
The correct chromosome for the ABO gene is chromosome 9. The ABO gene is located on the long arm of **chromosome 9** (9q34). This gene provides instructions for making an enzyme that helps determine an individual's ABO blood type by modifying the carbohydrate content of red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not correspond to the known location of the ABO gene.
**Option B:** Incorrect as it is not associated with the ABO blood group system.
**Option C:** Incorrect because, although it is involved in other genetic conditions, it is not the chromosome for the ABO gene.

**Clinical Pearl / High-Yield Fact**
Remembering the location of the ABO gene on **chromosome 9** is crucial for understanding genetic inheritance patterns of ABO blood types. This knowledge is vital in transfusion medicine to prevent adverse reactions.

**Correct Answer:** D. 9