In codominant inheritance, both alleles in a heterozygous organism are fully expressed. In this case, the homozygous white horse (CWCW) is bred with a homozygous black horse (CBCB).
The resulting offspring will all be heterozygous (CWCBC), which will represent both colors in their hair, exhibiting the roan phenotype.
Given that all offspring will be roan, the likelihood that their offspring will be roan is 100%.
So the correct answer is: 100%