Final answer:
Crossing over and random segregation occur during meiosis, specifically during prophase I and metaphase I respectively, contributing to genetic variation. Over 8 million chromosome combinations can result from this process, leading to a significant potential for diversity in offspring.
Step-by-step explanation:
Crossing over and random segregation, which produce genetic variation, occur during meiosis. Specifically, crossing over happens during prophase I of meiosis, where there is an exchange of genetic material between non-sister chromatids of homologous chromosomes, leading to new gene combinations. Additionally, during metaphase I of meiosis, random segregation, also known as independent assortment, occurs when chromosomes line up at the metaphase plate in a random order, resulting in over 8 million possible combinations of chromosomes for each gamete.
Regarding fertilization, it combines these varied gametes into a zygote, further contributing to genetic variation. Each unique sperm can fertilize any equally unique egg, leading to more than 64 trillion possible genetic outcomes. Therefore, the process that involves both crossing over and random segregation is meiosis (B).