During which phase of meiosis does crossing over of chromosomes occur?

-During pachytene stage of prophase I.

Explanation:

Crossing over can be defined as the process of exchange of genetic material of a cell between two homologous, non-sister chromatids. It results in formation of recombinant chromosomes and causes variations in offspring.

Crossing over takes place during phachytene stage of prophase I of meiosis, during which homologous chromosomes come close by synapis and exchange the genetic material.

Thus, the correct answer is 'pachytene stage of prophase I.'