Concept:Identical twins form when one egg fertilized by one sperm splits into two embryos.
Explanation:A single sperm fertilizes one egg, creating a zygote.
During early development, the zygote divides into two separate parts.
Each part develops into a baby with identical genetic material.
This is called monozygotic twinning.
The process begins with sexual reproduction (fertilization), but the splitting is like asexual reproduction.
Thus, identical twins result from one egg and one sperm.
Answer:Fertilization of one egg by one sperm (Option D).