Which type of chromosomal mutation causes down syndrome?

Not mutation,number . Down syndrome is an abnormality of chromosome numbers, not a trait or mutation in the gene code. During conception only one of chromosome of each pair is given by both parents to the fertile egg.If a parent adds their 2 nbr 21chm to the other parents nbr 21, 3 end up in the egg ; the excess 21 causes the all the problems.Rare forms are causes by a extra piece of a 21, but that's another issue.
Trisomy 21. Down syndrome is also called trisomy 21. That means that there is a partial or complete third copy of the chromosome 21. It is the most common chromosomal abnormality.