Final answer:
Fertilization is the process that transitions from haploid to diploid stages in the alternation of generations, forming a diploid zygote which develops into a multicellular diploid organism.
Step-by-step explanation:
The event that leads from the haploid stage to the diploid stage in the alternation of generations is fertilization. In sexual reproduction, haploid gametes (sperm and egg), each carrying one set of chromosomes, unite during fertilization to form a diploid zygote, which contains two sets of chromosomes. This diploid zygote then goes through mitosis to develop into a multicellular diploid organism. The alternation of generations cycle includes both haploid and diploid multicellular organisms. The diploid organism (sporophyte) will produce haploid spores by meiosis, and these spores eventually grow into a haploid organism (gametophyte), completing the cycle.