In a codominant inheritance pattern, when two homozygous individuals are bred, the offspring will exhibit a mixture of both traits.
In this case, the homozygous white horse (CWCW) and the homozygous black horse (CBCB) will produce offspring that are heterozygous (CWCB), which will express both the white and black traits. Therefore, the offspring will appear roan, exhibiting both colors in their coat.
Since all offspring will be CWCB, the likelihood that their offspring will be roan is 100%.
The correct answer is 100%.