Answer: D. offspring with red and green cones.
Explanation:
Codominance is a form of inheritance in which the alleles of a pair of genes in a heterozygous are fully expressed. As a result the offspring phenotype is a combination of the parents' phenotype. Therefore, the trait is neither dominant nor recessive.
In the codominant inheritance, two different versions so called alleles of a gene are expressed and each allele produces a somewhat different protein. Here, both versions influence the genetic trait or determine the characteristics of the genetic condition.