The correct answer is B. Be colour blind.

Since the boy has the genotype YXn:

- The Y chromosome indicates that he is male.

- The Xn allele is the recessive allele for colour blindness.

As a male, he only has one X chromosome, and since it carries the Xn allele, he will express the trait of colour blindness. Males are more likely to express X-linked recessive traits, such as colour blindness, because they have only one X chromosome. Females, on the other hand, would need to inherit two copies of the Xn allele (one from each X chromosome) to express colour blindness.

The other options are not correct:

A. The boy will not have normal vision because he has the Xn allele.

C. The boy will not be totally blind; colour blindness is a specific visual impairment, not total blindness.

D. The boy cannot be a carrier of colour blindness because he is male and only has one X chromosome. Carriers are typically females who have one Xn allele and one normal allele (XN).