<prompt> Most common trisomy among following is :1821135  <response>: 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 Downs 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. 