What is a zygote?

A zygote is defined as the union of a sperm and an ovum, which marks the initial stage of development in sexually reproducing organisms. This process occurs during fertilization, where the genetic material from the male and female gametes combines to form a single diploid cell. This cell, the zygote, contains all the genetic information necessary to develop into a new organism.

Understanding this definition clarifies why the other options do not accurately describe a zygote. The first cellular division of an embryo refers to a process that occurs after the formation of the zygote, indicating subsequent stages of embryonic development. A stage in a plant's life cycle does not pertain specifically to the concept of a zygote, as zygotes are primarily associated with the reproductive processes in both animals and plants, but not all stages in a plant's life cycle directly relate to a zygote. The final stage of fetal development refers to a later stage in the gestation process, which follows the zygote stage, further emphasizing that the zygote itself is the very first stage following fertilization.