Final answer:
A zygote is the one-cell structure created when a sperm and egg merge, marking the beginning of embryonic development(Option D).
Step-by-step explanation:
The structure that begins as a one-cell structure created when a sperm and egg merge is the option D: zygote.
After the sperm penetrates the egg, their nuclei fuse to form a diploid zygote, initiating the process of embryonic development. The zygote then undergoes rapid cell division, known as cleavage, and progresses through stages such as the morula and blastocyst, eventually implanting into the uterine wall where it continues to develop.