The best term to complete the set is fertilization.
Here's the reasoning:
- The sequence starts with "sperm" and "egg," which are the gametes involved in sexual reproduction.
- When these two gametes combine, they form a "zygote."
Fertilization is the process that leads to the formation of a zygote, so it logically fits into the sequence after sperm and egg, before reaching the zygote.
The other terms listed (binary fission, vegetative propagation, sporulation) refer to asexual reproduction methods and do not relate directly to the process of forming a zygote from the sperm and egg.
Therefore, "fertilization" is the term that best completes the set.