What is the cause of Down's syndrome?

A) Presence of additional copy of X-chromosome i.e. 47 with XXY.
B) Presence of additional copy of chromosome number 21.

C) Absence of one of the X-chromosomes i.e. 45 with XO.

D) Lack of enzyme that converts the amino acid phenylalanine and into tyrosine.

The correct answer is:
B) Presence of additional copy of chromosome number 21.