Final answer:
The only impossible genotype for the children of a colorblind woman and a man with normal vision is XBXB, as this would require the father to contribute a recessive colorblind allele, which he does not have.
Step-by-step explanation:
The question revolves around the inheritance of colorblindness, specifically the genotypes that are possible for the children of a colorblind mother and a father with normal color vision. Because colorblindness is an X-linked recessive condition, a colorblind woman's genotype is XBXB, and her husband, with normal vision, would be XBY. Their sons would inherit an XB from their mother and a Y from their father, resulting in XBY—a genotype for an affected son. Their daughters could inherit an XB from their mother and an XB from their father, which is not possible since the father has normal vision, meaning this genotype (XBXB) is incorrect for a daughter of this pair. A daughter would instead inherit XB from the mother and X from the father, resulting in XBX and becoming an unaffected daughter but a carrier of the trait. Therefore, the only impossible genotype for their children is option a. XBXB, as this would indicate a daughter who is colorblind, which cannot be the case given the normal vision of the father.