Based on the events described in the text:

1. Pachytene (a): Characterised by the appearance of recombination nodules and crossing over (3) between non-sister chromatids .
2. Metaphase I (b): The bivalent chromosomes align on the equatorial plate (4) .
3. Diakinesis (c): This stage is marked by the terminalisation of chiasmata (2) .
4. Zygotene (d): Chromosomes start pairing together, a process called synapsis which implies the first option (1) usually listed as 'Synapsis' or 'Pairing of homologous chromosomes' . Thus, the correct match corresponds to the sequence a-3, b-4, c-2, d-1.