Concept:Trisomy 21 means an individual has three copies of chromosome 21 instead of the normal two. This genetic condition directly causes Down syndrome.Explanation:In a normal human cell, there are 23 pairs of chromosomes. In Trisomy 21, an extra copy of chromosome 21 is present in every cell. This extra genetic material affects physical and mental development. Down syndrome is the most common example of an autosomal trisomy. It is named after Dr. Langdon Down, who first described it. Other options: Turner syndrome involves a missing X chromosome in females, and Klinefelter syndrome involves an extra X chromosome in males. Neither involves chromosome 21.Answer:Option A. Down syndrome