Final answer:
Crossing over is the event during the first meiotic prophase in which there is a reciprocal exchange of chromosome segments between homologous chromosomes.
Step-by-step explanation:
During the first meiotic prophase, the event that occurs and is known to be a reciprocal exchange of chromosome segments between homologous chromosomes is crossing over.
Crossing over occurs during prophase I of meiosis, when homologous chromosomes line up in pairs, known as tetrads. Non-sister chromatids of homologous chromosomes exchange genetic material, creating unique combinations of genes and increasing genetic variation.
This process is also called genetic recombination, and it plays a crucial role in producing gametes with different sets of alleles from the parents.