Crossing over, resulting in an increase in genetic variation, occurs between

nonsister chromatids of homologous chromosomes

Explanation:

solution

we know that crossing over  is the exchange of genetic material between the nonsister chromatids of homologous chromosomes  occurs during the pro phase

so increase in genetic variation occure bewteen nonsister chromatids of homologous chromosomes

correct answer is nonsister chromatids of homologous chromosomes

answer; postconventional;