Answer d. pairing of homologues
Ø During leptotene, homologous chromosomes pair and chromatin begins to condense.
Ø Proteins, including the meiosis-specific cohesin complex, form a structure called an axial element between sister chromatids .
Ø During zygotene, the pairing of homologous chromosomes is stabilized by the initiation of Double Strand Breaks repair via IH (interhomologue) recombination and the formation of proteinaceous bridges, which in most organisms is the SC (synaptonemal complex).