Final answer:
During meiosis I, crossing over occurs during prophase I. Sister chromatids remain joined and only separate during anaphase II of meiosis II. The event related to crossing over is essential for genetic variation.
Step-by-step explanation:
During meiosis I, several events occur which are crucial to the process of creating haploid gametes from a diploid cell. The correct answer to the event that happens during meiosis
The statements saying that sister chromatids separate or centromeres divide during meiosis I (options a & e) are incorrect, as they are characteristic of meiosis II. Options b and d are also incorrect as they incorrectly describe the timing and nature of chromosome and chromatid separation. Therefore, the correct answer is that crossing over occurs during meiosis I.