Answer:
The correct answer is A non disjunction during meiosis.
Explanation:
Nondisjunction occur when the sister chromatids does not separate during the course of cell division.As a result the daughter cells those are formed by non disjunction contain abnormal number of chromosomes.
Trisomy 21 or down syndrome is also caused by non disjunction as the 21st chromosome is not separated during the period of development.This ultimately result in the production of daughter cell containing extra copy of 21th chromosome.Thus the newly produced cell contain 47 chromosomes with a an additional third copy of chromosome 21.
