In a codominant inheritance pattern, both alleles in the genotype express themselves fully in the phenotype. In this case, the homozygous white horse has the genotype C^W C^W, and the homozygous black horse has the genotype C^B C^B.
When these two horses are bred together, all of their offspring will have the genotype C^W C^B. Since both alleles are codominant, both colors will be expressed in the phenotype of the offspring. Therefore, the offspring will have a coat that shows both white and black coloration.
Based on this information, the likelihood that their offspring will be born with both colors represented in their hair is:
A: 100%