Final answer:
Nondisjunction can occur during either anaphase I or anaphase II of meiosis, resulting in gametes with abnormal numbers of chromosomes.
Step-by-step explanation:
Nondisjunction is a process during which there is a failure of chromosomes to separate properly during cell division. This event can take place during meiosis I or meiosis II. When nondisjunction occurs in meiosis I, homologous chromosomes do not separate, leading to two gametes missing a chromosome and two with an extra chromosome. If nondisjunction happens in meiosis II, sister chromatids fail to separate, resulting in one gamete lacking a chromosome, two normal gametes, and one gamete with an extra chromosome.
Therefore, the correct answer to the question 'Nondisjunction occurs during what stage of meiosis?' is 'Either anaphase I or anaphase II'.