Final answer:
The Down syndrome critical region (DSCR) is a part of chromosome 21 that is associated with the characteristics of Down syndrome, making option 'a)' the correct answer.
Step-by-step explanation:
The Down syndrome critical region (DSCR) is a specific segment of chromosome 21 that contains genes thought to contribute significantly to the characteristics individuals with Down syndrome commonly exhibit. The correct answer to the question 'What is the Down syndrome critical region (DSCR)?' is a) A region on chromosome 21 associated with Down syndrome. Down syndrome, also known as Trisomy 21, occurs when a person has three copies of chromosome 21, rather than the usual two, due to a nondisjunction event during meiosis. This additional genetic material alters the course of development and causes the varied characteristics of Down syndrome, such as cognitive impairment, a specific facial appearance, and other health-related issues.