Quantitative inheritance is defined as inheritance of a character which is controlled by polygenes.
A. Human skin colour is a polygenic trait as it is controlled by genes A, B and C. Each allelic pair has a quantitative effect on the phenotypic expression.
B. Colourblindness is a X-linked recessive trait caused by the gene located on X-chromosome.
C. Klinefelter's syndrome is a genetic disorder caused due to abnormality in sex chromosome number. the affected individual has 47 chromosomes (47 + XXY).
D. Alkaptonuria is an inherited metabolic disorder which occurs due to deficiency of an oxidase enzyme required for the breakdown of homogentisic acid.
Hence, example of quantitative inheritance is colour of skin. So, the correct answer is 'Colour of skin'.