Final answer:
A zygote is a diploid cell that results from the fertilization of two haploid gametes, containing a full set of chromosomes from both parents. It is the initial cell that undergoes mitosis to develop into a multicellular organism.
Step-by-step explanation:
The zygote is defined as a cell that forms as the result of fertilization and contains the same number of chromosomes as the parent cells. This describes option D accurately. During reproduction, two haploid gametes from each parent fuse together to create the zygote, which is a diploid cell. This single cell contains a full set of chromosomes, half from each parent. In humans, a zygote would have 46 chromosomes, comprising 23 pairs. The zygote then begins to divide by mitosis to eventually form a multicellular organism. The process of going from a single-celled zygote to a full organism, such as a human, involves billions of cell divisions and is the beginning of embryonic development.