The answer is B) Chromatids of homologous chromosomes exchange corresponding segments.
The crossover of chromosomes is the exchange of
genetic material between homologous chromosomes. Homologous chromosomes are present in the meiosis I, but not in meiosis the II. After pairing up of in the prophase I, homologous chromosomes are able to exchange some corresponding segments and a cross-over occurs. Thanks to the cross-over event, each chromosome will have a new combination of genes. Without this, parents will produce gametes with only two different gene combinations.
