Final answer:
In sexual reproduction, the process that unites gametes and restores the diploid number of chromosomes is called fertilization. It occurs after the production of haploid gametes by meiosis and results in the formation of a zygote.
Step-by-step explanation:
During sexual reproduction, the process in which gametes unite to restore the diploid number of chromosomes is called fertilization. Sexual reproduction involves the production of haploid gametes through a process known as meiosis, which includes the reduction of the chromosome sets by half. When two haploid gametes from different parents fuse during fertilization, they form a zygote, which is a diploid cell, containing two sets of chromosomes. This ensures the continuation of a species while preventing an exponential increase in the chromosome number with each generation.