The answer is meiotic prophase I.
Since Sheryl is observing ovarian slides showing crossover chromosomes it is expected that she is observing meiosis and not mitosis. 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 Sheryl is observing meiosis I. Now, we need to assume in which of the phases of meiosis I. Homologous chromosomes pair up in the prophase. Therefore, meiosis prophase I is the phase in which chromosomal crossover occurs.
