Which of the following is a sex-linked disease ?

The correct answer is Colour blindness.

Explanation:

• Sex-linked diseases are genetic disorders that are associated with the sex chromosomes, primarily the X chromosome. These disorders are often passed down from parents to offspring.
• Colour blindness is a common example of an X-linked recessive disorder. It is caused by mutations in the genes responsible for the production of photopigments in the eye.
• It is due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour.
• This defect is due to mutation in certain genes present in the X chromosome.
• It occurs in about 8 per cent of males and only about 0.4 per cent of females.
• Since males have only one X chromosome (XY), they are more likely to exhibit X-linked disorders.
• Females (XX), on the other hand, need mutations in both X chromosomes to express the condition, making them less likely to be affected.

Other Options:

• Myopia: Myopia, or nearsightedness, is a refractive error of the eye that causes difficulty in seeing distant objects clearly. 
• Smallpox: Smallpox is a viral infection caused by the variola virus. It has been eradicated globally through vaccination programs
• Scurvy: Scurvy is a nutritional deficiency disease caused by a lack of vitamin C (ascorbic acid) in the diet.