What is a zygote?

A zygote is the initial stage of human development that begins when a sperm cell fertilizes an egg cell. This marks the very first moment of a new human organism's life. Once the sperm and egg unite, the zygote contains all the genetic information necessary to develop into a complete human being.

Although the other options may involve aspects of reproduction or biology, they do not define a zygote specifically. Reproductive cells refer to sperm and egg, which are involved in the process leading to the formation of a zygote, while somatic cells pertain to all body cells that are not involved in reproduction. Chromosomes are the structures within the cells that contain genetic material, which zygotes possess, but they do not define what a zygote itself is. By understanding that a zygote is the first developmental stage, it becomes clear why this is the correct answer.