AB blood type is an example of what type of inheritance? Both alleles show in the phenotype

The correct answer is: Co-dominance

Codominance, as it relates to genetics, refers to a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual. That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or animal that has more than one pigment color.
In people, one codominant trait is blood type. People with the AB blood type have one A allele and one B allele. Because both alleles are expressed at the same time, their blood type is AB.