"Nondisjunction occurs when
1. a portion of a chromosome breaks off and is lost.
2. two chromosomes fuse into one.
3. members of a chromosome pair fail to separate.
4. an entire pair of chromosomes is lost during meiosis I."

The correct answer is option 3.

Explanation:

When the chromosomes fail to segregate accurately, which results in the formation of gametes with one missing or additional chromosome is termed as non-disjunction. In the process of non-disjunction, the sister chromatids or the homologous chromosomes fail to segregate accurately at the time of cell differentiation.  

This failure of homologous chromosomes to separate may take place due to the failure of sister chromatids to separate at the time of Anaphase I that results in the formation of four deformed daughter cells, or during Anaphase II stage that results in the formation of two affected daughter cells.