Mongolian idiots are due to trisomy in 21${}^{st}$ chromosome is called as

The correct option is A Down's syndrome
Trisomy mainly exists due to one extra copy of chromosome which is caused due to certain mutation or by non-disjunction of homologous chromosomes during cell division. The term Mongolian idiocy have been used to refer to a specific type of mental deficiency associated with the genetic disorder, more commonly referred to as Down syndrome.
Therefore, the correct answer is option A.