Down's syndrome is due to trisomy of 2121st chromosome caused by

The correct option is D Either A or B

The most common form of Down syndrome is known as trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. Trisomy 21 is caused by an error in cell division called nondisjunction. This leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.

So, the correct answer is 'Either A or B'.