In genetics, when you have a homozygous white horse (WW) and a homozygous black horse (BB) bred together, the offspring will are all heterozygous (WB) and express only the dominant color (in this case, it depends on the dominance relationship of the alleles for coat color).
Since none of the options provided suggest a straight-forward dominance relationship indicating roan horses from the given parental genotypes, and considering the standard understanding of coat colors in horses:
If we were to incorporate what roan typically means (a coat color pattern that is a mix of white and color hairs), this usually doesn't arise from straightforward black and white crosses unless there's a specific roan gene. The roan phenotype typically arises from specific alleles that mix colors, such as red and white or chestnut and white.
Following standard Mendelian inheritance:
- Offspring from a WW (white) and BB (black) would be all black (as the black is dominant), which means that no offspring would be roan.
Thus, the correct answer to the original question, based on standard understanding, would be: 0 percent.