Which type of chromosomal mutation causes this form of down syndrome?

Trisomy 21 (nondisjunction) Down syndrome is usually caused by an error in cell division called "nondisjunction." Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.

Nondisjunction is the chromosomal mutation