Recombination involves :

Cross-over between non-sister chromatids.

The correct answer is Option (3) -Cross-over between non-sister chromatids .

Chromosomal crossover is the exchange of genetic material between homologous chromosomes, leading to the formation of recombinant chromosomes. This process occurs during prophase I of meiosis, specifically in the pachytene stage.

During this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.

Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.