Which type of mutation causes Down syndrome?
I'm not an expert on this, but I don't think it is considered a mutation. It is caused by an extra chromosome (non-mutated chromosome).
Down Syndrome is caused by a condition called trisomy 21, or 3 copies of chromosome 21. I don't think this arises from a mutation, but rather a problematic step in meiosis.
This answer might not be complete, but gives a place to start looking from.