Final answer:
The transition from haploid to diploid in life cycles takes place during fertilization.
Step-by-step explanation:
The transition from haploid to diploid takes place during fertilization. Fertilization is the process in which the haploid gametes, one from each parent, fuse together to form a diploid zygote. This zygote then undergoes mitosis and cell division to develop into a multicellular diploid organism.