## Core Concept
The ABO blood group system is a classification of blood types based on the presence or absence of specific antigens on the surface of red blood cells. These antigens determine an individual's blood type according to the ABO blood group system, which is controlled by a single gene with three types of alleles: A, B, and O. The expression of these alleles follows a specific pattern of inheritance.

## Why the Correct Answer is Right
The correct answer, , represents multiple alleles and the concept of codominance. In the ABO blood group system, the A and B alleles are codominant, while the O allele is recessive. This means that both A and B alleles will be expressed if an individual has both (AB blood type), illustrating codominance. The presence of three different alleles (A, B, and O) for the same gene makes it an example of multiple alleles.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option is incorrect because ABO blood group inheritance does not demonstrate incomplete dominance. Incomplete dominance would result in a blending of the phenotypes, which is not observed in the ABO blood group system.
* **Option B:** - This option is incorrect because while ABO blood group inheritance does involve multiple alleles (A, B, and O), it is also characterized by codominance, which is not mentioned here.
* **Option C:** - This option might seem plausible because it mentions codominance but fails to acknowledge the presence of multiple alleles.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the ABO blood group system is clinically significant in transfusion medicine. Individuals with AB blood type are considered universal recipients because they have no A or B antibodies, but they can only donate to AB individuals. This system is also important in understanding certain diseases and in forensic science.

## Correct Answer: . Multiple alleles and codominance