Most common trisomy among following is :

Answer is B (Trisomy 21) : The most common abnormalities of chromosome number are `trisomies' - Nelson The most frequent and best known trisomy in humans is trisomy 21 or Down's syndrome. - Nelson This is the commonest chromosomal disorder occurring with a frequency of 1:800 to 1:1000 new borns. Down syndrome occurs more often in offspring of mothers conceiving at older age.