Down's syndrome is caused by:

trisomy of $14^{\text {th}}$ chromosome trisomy of $22^{\text {nd}}$ chromosome trisomy of $21^{\text {st}}$ chromosome short arm of $23^{\text {rd}}$ chromosome

trisomy of $21^{\text {st}}$ chromosome

The correct answer is Option (3) → trisomy of $21^{\text {st}}$ chromosome