Crossing-over takes place in the

The correct option is Pachytene stage.

To understand why crossing-over occurs during this specific stage, let's break down the phases of Prophase I of meiosis:
1. Leptotene: Chromosomes begin to condense and become visible under the light microscope.
2. Zygotene: Homologous chromosomes start pairing up in a process called synapsis, forming a complex structure called a synaptonemal complex.
3. Pachytene: During this stage, the paired homologous chromosomes (called bivalents or tetrads) become clearly visible. This is the crucial phase where non-sister chromatids of homologous chromosomes exchange genetic material. This exchange process is known as crossing-over, which is facilitated by the recombination nodule.
4. Diplotene: The synaptonemal complex dissolves, and the homologous chromosomes begin to separate except at the sites of crossovers, forming X-shaped structures called chiasmata.
5. Diakinesis: This is the final stage of Prophase I, characterized by the terminalisation of chiasmata and the preparation of chromosomes for segregation in metaphase.

Since the physical exchange of genetic segments (crossing-over) occurs during the third stage of Prophase I, the Pachytene stage is the correct answer.