A boy with normal brother and colourblind sister has his parents
A) Father normal, mother colourblind
B) Both normal
C) Both colourblind
D) Father colourblind, mother normal

Hint: Color blindness is the disease in which a person is unable to distinguish one or more colors of the eyes, and it is sex-linked genetic diseases that mainly affect males mostly than females.

Complete answer:
Generally in humans, this disease is inherited, where the genes that are present on the cones of eyes that are responsible for color vision are defective.
This color blindness X-linked disease, where that color-blind defective gene is transferred from parents to offspring.
In the above question, they mentioned that the children of parents have a normal boy and colorblind girl, as we know that it is X linked, so in males, only one X chromosome is present which means one gene causes the disease if it is dominant.
So if the mother is color blind, it definitely passes to the boy, and he should be colorblind, but in question, the boy is normal which indicates that the mother is normal.
And they mentioned that normal boy sister is colorblind, in females they get one X chromosome from mother and one from father, as we already know that mother is normal, but girl is colorblind means she got it from father who is color blind, so that the parents of those children are color blind father and normal mother.

So, the correct option is D.

Note: Color blindness are mainly occurs in males, it is because they have only X chromosome, the gene responsible for color blindness if transmitted, there will be phenotypic expression, but for the female to produce the color blindness, where both X chromosomes should have dominant genes for color blindness, even single makes them carrier but not shows the phenotypic expression.