When a chromosome pair fails to separate correctly in meiotic division, what is it called?
a. chromosome deletions
b. crossing-over
c. nondisjunction
d. trisomy

C. Nondisjunction. 
Nondisjunction is the failure of  one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division. This can result in deformities in a new-born such as Down Syndrome, due to lack of chromosomes in the body.

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