The answer is C) crossing over.
The crossover of chromosomes is the exchange of genetic material between homologous chromosomes. If it is known that homologous chromosomes are present only in the meiosis I, it is expected that crossover is occuring meiosis I. Homologous chromosomes pair up in the prophase. Therefore, meiosis prophase I is the phase in which chromosomal crossover occurs. Chromosomal crossover is process that helps make meiotic cells genetically different.