Crossing over occurs during
a)	Leptotene	b)	Diplotene	c)	Pachytene	d)	Zygotene

(c)

Crossing over occurs during pachytene or thick thread or pachynema substage of prophase-I of meiosis. During this stage, an exchange of portions of chromatids between homologous chromosomes occur. At chiasma, the chromatids break rejoin in such a way that sections are exchanged.