Ans. D. Trisomy 21An extra chromosome 21 is the most common genetic abnormality found in Down syndrome, and the abnormality most likely to cause mental retardation. Abnormalities in autosomal chromosomes are, in general, associated with mental retardation. The chromosomal aberration represented by 46 chromosomes with fusion of 15 and 21 produces a type of Down syndrome that, unlike trisomy 21, is usually inherited. Aberrations in sex chromosomes are not always associated with mental retardation, such as in XO (Turner's syndrome), XXY (Klinefelter's syndrome), and XXYY and XXXY (Klinefelter's syndrome variants) genotypes.Some children with Turner's syndrome have normal to superior intelligence.Girls with Turner's syndrome have gonadal agenesis and do not develop secondary sexual characteristics without medical intervention. Another hallmark feature is a webbed neck. In Klinefelter's syndrome and its variants, individuals have underdeveloped male genitalia and infertility and may develop gynecomastia beginning in adolescence.