Down syndrome occurs when an individual has three copies of chromosome 21 instead of the normal complement of two copies of this chromosome. Which
statement best explains how this condition can result?
a) improper crossing over during meiosis I,
b) mutation of chromosome 21 after meiosis is completed,
c) lack of spindle fiber attachment to centromeres during meiosis I,
d) lack of separation of homologous chromosomes during meiosis II.