**Core Concept**
ABO antigens are complex carbohydrate molecules expressed on the surface of red blood cells, playing a crucial role in blood transfusion compatibility. The presence or absence of these antigens determines an individual's ABO blood group, with four main groups: A, B, AB, and O.

**Why the Correct Answer is Right**
The ABO antigens are encoded by the ABO gene, which is located on chromosome 9. The gene has three alleles: A, B, and O. The A and B alleles encode enzymes responsible for adding N-acetylgalactosamine (GalNAc) or N-acetylglucosamine (GlcNAc) to the H antigen on the red blood cell membrane, respectively. The O allele, on the other hand, encodes a truncated enzyme that prevents the addition of these sugars, resulting in the H antigen only.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because ABO antigens are indeed present on red blood cells, as well as in some other bodily fluids such as saliva and semen.
**Option B:** Incorrect because ABO antigens are not present in significant amounts in the fetus before 28 weeks of gestation, but this option is not the correct choice.
**Option C:** Incorrect because ABO antigens are not present in the fetus before 28 weeks of gestation, this is not the correct choice.
**Option D:** Incorrect because ABO antigens are not present in the fetus before 28 weeks of gestation, this is not the correct choice.

**Clinical Pearl / High-Yield Fact**
ABO incompatibility is a major concern in blood transfusions, as it can trigger a severe hemolytic reaction. Always verify the ABO blood group of both the donor and recipient before transfusing blood products.

**Correct Answer:** D.