Final answer:
The union of gametes at fertilization restores the diploid number of chromosomes. When a haploid sperm and a haploid egg combine, they form a diploid zygote. The correct multiple-choice answer to the given question is c. the diploid number of chromosomes.
Step-by-step explanation:
The union of gametes at fertilization restores the diploid number of chromosomes. During gametogenesis, the process that produces gametes, the chromosome number is halved to produce haploid gametes. This reduction is necessary so that when fertilization occurs, the joining of a haploid sperm and a haploid egg can restore the diploid condition, which is characteristic of the species (typically 2n in chromosome number; in humans, this is 46 chromosomes).
Therefore, the correct answer to the question is c. the diploid number of chromosomes. It is important to note that fertilization does not necessarily guarantee the viability of the embryo, nor does it involve the merging of two diploid cells into a haploid cell. Instead, it takes one haploid cell from each parent to form a new diploid organism. Sexual reproduction includes meiosis, which reduces the diploid cells to haploid cells, and fertilization, where two haploid gametes unite to restore the diploid number of chromosomes.