The stage of prophase I wherein crossing over occurs is

The correct option is Pachytene.

Prophase I is the first stage of meiosis I and is characterized by a series of complex chromosomal events. It is divided into five distinct substages to describe these events chronologically: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

During the Leptotene stage, chromosomes begin to condense and become visible under the light microscope.
During the next stage, Zygotene, homologous chromosomes start pairing up side-by-side in a process called synapsis, mediated by the formation of a complex structure known as the synaptonemal complex.
Once synapsis is complete, the cells enter the Pachytene stage. During this stage, the paired homologous chromosomes (each consisting of a pair of sister chromatids, forming a bivalent or tetrad) become clearly visible. It is during pachytene that the non-sister chromatids of homologous chromosomes exchange genetic material. This exchange process is known as crossing over, which is facilitated by the recombination nodule. Crossing over leads to genetic recombination, which is a major source of genetic variation in sexually reproducing organisms.
Following pachytene, the Diplotene stage begins, marked by the dissolution of the synaptonemal complex and the separation of the homologous chromosomes except at the sites of crossovers, called chiasmata.
Finally, during Diakinesis, the chiasmata terminalize, the nucleolus disappears, and the nuclear envelope breaks down.

Therefore, the specific stage of prophase I where crossing over occurs is Pachytene.