Question

when does nondisjunction occur?
when a nucleotide is deleted
when homologous chromosomes fail to separate during meiosis
when a piece of a chromosome reverses

Explanation:

Nondisjunction is the failure of homologous chromosomes or sister - chromatids to separate properly during cell division, most commonly during meiosis. A nucleotide deletion is a type of mutation, and a chromosome piece reversing is inversion.

Answer:

when homologous chromosomes fail to separate during meiosis