Trisomy of Chromosome 21 in humans leads to:

The correct answer is Option (3) → Down's Syndrome

Option (3) Down's Syndrome — Correct. Down’s syndrome is caused by an additional copy of chromosome 21, called trisomy of 21. Those affected are short, with a small round head, a furrowed tongue and partially open mouth.

NCERT Extract:
Chromosomal Disorders – “… 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 (Figure 4.16). Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.”