Question

Down's syndrome involves
a. sex-linked trait, gene mutation, chromosomal abnormality

Answer 1

It involves chromosomal abnormality; when the condition is caused by trisomy 21, (down syndrome), the chromosomal abnormality occurs as a random event during the formation of reproductive cells in a parent. The abnormality usually occurs in egg cells, but it occasionally occurs in sperm cells.
I hope this answers your question. =)