About an hour or so after the sperm enters the ovum, the two gametes suddenly fuse, becoming a single cell organism called a(n)

The correct answer is Zygote

Explanation:

Upon mating, sperm from the male enters the ovum of the female, then two gametes suddenly fuse to form a FERTILIZED SINGLE CELL called ZYGOTE.

This zygote on further development becomes an embryo, and then a foetus which could be delivered at the end of a gestation period.

gametes or germ cells are haploid cells (example: sperm and ova) containing only one set (or n) number of chromosomes and autosomal or somatic cells are diploid cells containing 2n number of chromosomes.