The correct option is C Assertion is correct but Reason is incorrect
Crossing over or recombination occurs during pachytene. Recombination
involves mutual exchange of the corresponding segments of non-sister
chromatids of homologous chromosomes. It takes place by breakage and
reunion of chromatid segments. Breakage, called nicking, is assisted by
an enzyme endonuclease and reunion, termed annealing, is aided by an
enzyme ligase. Crossing over leads to recombination of genetic material
on the two chromosomes. So, the correct answer is 'Assertion is correct but Reason is incorrect'.