, Klinefelter's Syndrome is a chromosomal disorder in humans (specifically males, 'in man') caused by the addition of an extra X-chromosome, resulting in a karyotype of 47, XXY. While the text primarily highlights that such individuals are sterile and have gynaecomastia (feminine development) , chromosomal disorders involving aneuploidy like Trisomy 21 (Down's syndrome) are the classic cause of mental retardation . However, among the given options, the 'addition of one X chromosome' (Klinefelter's) is the only viable chromosomal abnormality listed that affects males (XY becoming XXY). In broader medical genetics, Klinefelter's can be associated with some cognitive impairment, distinguishing it from Turner's (Option 1, Loss of X) which affects females.