Answer:
Meiosis
Step-by-step explanation:
Diploid sporophyte cells go through meiosis to supply haploid spores. Each spore undergoes mitosis to supply a multicellular haploid gametophyte. Mitosis inside the gametophyte is needed to generate the gametophyte. A diploid sporophyte outcomes from the fusion of gametes. (None of this is plagiarized you can use it if you want or need)