Answer: Mitosis
Step-by-step explanation:
1. Sporophyte produces haploid spores by meiosis
2. Spores germinate and divide by mitosis and develop into multicellular, haploid gametophytes
3. Gametophytes produce unicellular haploid gametes by mitosis
4. Two gametes unite during fertilization to form a diploid zygote
5. Zygote divides by mitosis and develops into a multicellular, diploid sporophyte.