Final answer:
Crossing-over occurs during prophase I of meiosis I, where homologous chromosomes pair up and exchange genetic material, resulting in genetic recombination.
Step-by-step explanation:
The question pertains to the process of crossing-over, which is a key event during meiosis. Crossing-over usually occurs during prophase I of meiosis I, where homologous chromosomes pair up and exchange genetic material. This exchange of genetic material between non-sister chromatids of homologous chromosomes leads to genetic recombination, which is crucial for creating genetic diversity.
During prophase I, homologous chromosomes come together to form a tetrad - a group of four chromatids. It's within this tetrad that crossing-over, or the exchange of DNA segments between non-sister chromatids, takes place. This recombination event can happen multiple times within the same homologous pair, producing unique combinations of genes on each chromosome.
In the context of the question regarding when crossing-over occurs, the answer is: a. prophase of meiosis I. None of the other options provided are stages during which crossing-over occurs.