Crossing over occurs during d) prophase I of meiosis.
During prophase I, homologous chromosomes pair up in a process called synapsis. As they come together, sections of chromatids can exchange places through a process known as crossing over. This exchange of genetic material is crucial as it increases genetic diversity in the gametes that are produced. By the end of prophase I, the chromosomes may have different combinations of alleles, contributing to genetic variation in the offspring.