If two alleles are codominant,
(C) is the correct answer. When two alleles are codominant, both will express themselves in a mixed phenotype. For example, a roan cow will have splotches of red and white on the coat. You may have been tempted to pick (D) but this describes incomplete dominance. For example, in flower color, a red and white allele will create a pink phenotype, but you will not have a pink cow.