______ is caused by the presence of an extra chromosome number 21.

The correct option is B Down syndrome
Down syndrome is a genetic disorder which arises due to the presence of an extra chromosome number 21 in every cell of the body. So, 47 chromosomes are present instead of the normal number 46. Down syndrome is also known as trisomy of the 21st chromosome. Children suffering from this disorder are usually mentally retarded and have a shorter life span of about 16–20 years.