Correct Answer - A
Croosing over a recombination occurs during pachytene. Recombination involves mutual exchange of the corresponding segments of non-sister chormatids of homologous chromosomes. It takes place by breakeage and reunion of chromatid segments. Breakeage, called nicking, is assisted by an enzyme endonulease and renuion, termed annealing, is aided by a nenzyme ligase. Crossing over leads to recombination of genetic material on the two chromosmes.