Is down syndrome autosomal or sex linked?

None of the above. Down syndrome happens when an extra 21 chromosome is present in all the cells of the body. It happens before conception when the donor egg fails to shed one of its 21 chromosomes in the formation of the egg; the male 21 joins the female pair & creates an embryo with 3 number 21's. It has nothing to do with traits or mutations in the gene code.
Extra chromosome. Down Syndrome is caused by having an extra copy of Chromosome 21, so instead of 2 copies, you have 3. This is an autosomal disorder and is not sex linked. This usually occurs due to abnormal cell division when they egg and the sperm meet during fertilization.