Down's syndrome is due to nondisjunction of

The correct option is C Autosome

Down syndrome is usually caused by an error in cell division called non-disjunction. Nondisjunction is a faulty cell division, which results in an embryo with three number 21 chromosomes instead of two. Prior to, or at, conception, a pair of number 21 chromosomes, in either the sperm or the egg, fail to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This faulty cell division is responsible for 95 per cent of all cases of Down syndrome.

So, the correct answer is 'Autosome'.