What Is the Basis of Codominance

What is the basis of codominance?

A) There is only one allele for a gene.
B) One allele of a gene encodes a protein, and the other allele is nonfunctional.
C) One gene has many alleles.
D) Each allele of a gene produces a protein that functions to create a trait.
E) One allele can have many effects on a phenotype.