Here's why:

* Haploid means having only one set of chromosomes.

* Diploid means having two sets of chromosomes.

During meiosis, a process of cell division specific to germ cells (cells that give rise to gametes), a diploid cell (in this case, an oocyte) divides into four haploid cells. One of these cells becomes the mature ovum (egg cell), while the others become polar bodies.

The ovum's haploid nature ensures that when it fuses with a sperm cell (also haploid), the resulting zygote will have the correct diploid number of chromosomes.