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The scenario showcasing codominance is found in the expression of blood type AB, where both A and B antigens are equally present on the surface of red blood cells. In codominance, both alleles in a heterozygous genotype are fully expressed, resulting in offspring that display distinct traits of both parents simultaneously. In this case, a person with blood type AB has inherited one A allele from one parent and one B allele from the other. Rather than one allele masking the other, as seen in complete dominance, both A and B traits are independently expressed, which is characteristic of codominance.

In contrast, the other scenarios illustrate different genetic principles. The mixing of red and white flowers to produce a pink flower exemplifies incomplete dominance, where the traits blend rather than appearing separately. A person with type O blood represents a case of complete dominance where the recessive allele (O) is not expressed at all, as O lacks antigens that are present in types A and B. Finally, a plant with a trait completely masked indicates complete dominance where one trait is fully hidden and does not manifest alongside the dominant trait. Thus, the blood type AB scenario is the clear representation of codominance in genetics.