Here's why:

* Fertilization is the process where a sperm cell (haploid, meaning it has half the number of chromosomes) fuses with an egg cell (also haploid).

* Diploid cells have two sets of chromosomes, one from each parent.

* The zygote, formed by the fusion of the sperm and egg, inherits one set of chromosomes from each parent, resulting in a full set of chromosomes. This makes the zygote a diploid cell.

Therefore, the zygote is the product of fertilization and is a diploid cell.