Haemophilia is more common in males because it is a.

The correct option is D Recessive trait carried by X-chromosome

As a recessive X-linked genetic disorder, the mutation that causes hemophilia is passed to offspring via the X chromosome. Hemophilia is more common among male children because they only inherit one X chromosome. Humans have 22 pairs of autosomal chromosomes and one pair of sex chromosomes, making a total of 46 chromosomes in each cell. In males, there is both an X chromosome and a Y chromosome, whereas females have two X chromosomes. Male offspring inherit their X chromosome from their mother and their Y chromosome from their father, while female offspring inherit an X chromosome from each parent. Since males only have one X chromosome, they will develop symptoms of hemophilia if that chromosome carries the mutation. However, because the condition is recessive, the inheritance of one abnormal X chromosome does not usually cause symptoms in females because they usually also inherit another normal X chromosome. These females are then classified as carriers of the disease and can pass on the mutation to male offspring even though they do not develop symptoms themselves. Technically, a female can develop hemophilia if she is born to a female carrier and a man affected by hemophilia, but this is very rare.

So, the correct answer is 'Recessive trait carried by X-chromosome'.