Final answer:
Fertilization leads to a diploid cell in a life cycle, as it is the union of two haploid gametes that results in a diploid zygote.a) Fertilization
Step-by-step explanation:
The event that leads to a diploid cell in a life cycle is fertilization. During fertilization, two haploid gametes, which carry a single set of chromosomes, unite to form a diploid zygote. This zigote, now containing two sets of chromosomes, represents the diploid stage of an organism's life cycle. In contrast, meiosis is the process that reduces the chromosome number by half to produce haploid gametes, and it involves two sequential stages: meiosis I and meiosis II. Gametogenesis is the process in which these haploid daughter cells mature into gametes, such as sperm and eggs. Most animals, including humans, exhibit a diploid-dominant life cycle, spending the majority of their lifecycle in the diploid phase, only producing haploid gametes during reproduction.