Down's syndrome is a result of gain of extra copy of chromosome number:

21

The correct answer is Option (1) → 21

Down’s Syndrome : The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21).

This disorder was first described by Langdon Down (1866). The affected individual is short statured with small round head, furrowed tongue and partially open mouth.

Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.