Concept:Crossing over is the exchange of genetic material between homologous chromosomes.Explanation:Crossing over happens during prophase I of meiosis.At this stage, paired homologous chromosomes exchange corresponding segments of DNA.This exchange creates new combinations of alleles on the chromosomes.As a result, the four daughter cells produced by meiosis are genetically different.This process does not occur during mitosis, fertilization, or mutation.Answer:D. meiosis