Crossing-over, also known as genetic recombination, is an important event that occurs during the process of meiosis. It involves the exchange of genetic material between homologous chromosomes, resulting in the creation of new combinations of alleles.

Option 1 - Leptotene: Leptotene is the first stage of prophase I of meiosis. It is characterized by the condensation of chromosomes. Crossing-over does not occur during this stage.

Option 2 - Zygotene: Zygotene is the second stage of prophase I. During this stage, homologous chromosomes pair up in a process called synapsis. Crossing-over can occur during this stage, as the paired chromosomes come into close proximity.

Option 3 - Pachytene: Pachytene is the third stage of prophase I. It is the longest stage and is marked by the further condensation of chromosomes. During this stage, crossing-over takes place, allowing for the exchange of genetic material between the paired chromosomes.

Option 4 - Diplotene: Diplotene is the fourth stage of prophase I. It is characterized by the separation of homologous chromosomes, except at areas where crossing-over has occurred. At these sites, the chromosomes may still be connected