A colour blind son will born whena. Mother is normal band father normal b. Mother is colour blind and father normal c. Mother is normal and father is colour blind d. All the cases are correct

Sure, here are the step-by-step solutions: Given: All the sons of a colourbind mother are colourblind. To prove: None of the sons of a colourblind mother are not colourblind. Proof: Let us assume that there exists a son of a colourblind mother who is not colourblind. Then, the given statement "All the sons of a colourbind mother are colourblind" would be false. But the given statement is true. Therefore, our assumption that there exists a son of a colourblind mother who is not colourblind is false. Hence, none of the sons of a colourblind mother are not colourblind.