The chromosomal disorders are caused due to absence, excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division results in the gain or loss of chromosome(s), called aneuploidy.

The disorder shown in the figure is caused due to:

Gain of an autosome

The correct answer is Option (4) → Gain of an autosome

Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Instead of the typical two copies of chromosome 21, individuals with Down's syndrome have three copies, leading to a total of 47 chromosomes in their cells instead of the usual 46. So Down's syndrome is caused due to the gain of an autosome.