In incomplete dominance, the phenotype of the offspring is a blend of the phenotypes of the parents. In this case, crossing a black chicken (BB) with a white chicken (WW) would result in offspring that exhibit a mix of the two colors.
Given the black (BB) and white (WW) phenotypes, the expected phenotype of the offspring would be gray (BW), as they would inherit one black allele and one white allele, resulting in a blended gray appearance.
Thus, the correct response is:
All gray