Most common chromosomal syndrome is:

c. Trisomy 21(Ref: Nelson's 20/e p 618-619)Down syndrome (Trisomy 21) is the most common of the chromosomal disorder.Trisomy 16 is the most common trisomy overall, however most do not survive and die in utero Hence, among the live births, or among children, most common chromosomal abnormality is Trisomy 21