Expansion of trinucleotide repeat
Correct answer :d Nondisjunction
Nondisjunction during the first meiotic division of gametogenesis accounts for most (92% to 95%) patients with Down syndrome who have trisomy 21.
The extra chromosome 21 is of maternal origin in about 95% of Down syndrome children.
Translocation of an extra long arm of chromosome 21 to another acrocentric chromosome causes about 5% of cases of Down syndrome. The other choices are unrelated to trisomy 21