Final answer:
A gamete is a reproductive cell containing one half of the chromosomes needed to produce an offspring; when two gametes unite, they form a zygote.
Step-by-step explanation:
The term that refers to a reproductive cell that contains one half of the chromosomes required to produce an offspring is gamete (Option 1). Gametes, such as sperm and egg cells in humans, are haploid cells, meaning they contain only one set of chromosomes, 23 in the case of humans. These haploid cells are produced through a special type of cell division known as meiosis. When a sperm and egg cell unite during fertilization, they form a zygote, which is a diploid cell having two sets of chromosomes, one from each parent. As the zygote develops, it will undergo multiple rounds of mitosis to produce a multicellular offspring. The terms zygote, embryo, and fetus refer to different stages in the development post-fertilization.