Down’s syndrome is due to trisomy of which of the following chromosom…

The correct answer is Option (4) → Chromosome 21

Down’s syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number 21, so the person has three copies of it instead of two (trisomy of 21). It arises from failure of segregation of chromatids during cell division, which is aneuploidy. The affected individual is short statured with a small round head, a furrowed tongue and a partially open mouth, and physical, psychomotor and mental development is retarded.

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.”

Figure 4.16 A representative figure showing an individual inflicted with Down’s syndrome and the corresponding chromosomes of the individual