The correct answer is d. formation of a zygote.
Explanation: Sexual reproduction involves the combination of genetic material from two parent organisms. This usually involves the fusion of specialized cells called gametes to form a zygote. The zygote is the first stage in the development of a new individual, containing genetic information from both parents.
In contrast, the other options represent forms of asexual reproduction. For instance, the formation of a bud (option a) is a process where a new organism develops from an outgrowth or bud on the parent organism. Fission (option b) is when a single organism divides into two or more parts, each growing into a new individual. Fragmentation (option c) involves the breaking of a parent organism into fragments, where each fragment can develop into a new organism. None of these processes involve the combination of genetic material from two parents, thus making them asexual forms of reproduction.