How are identical twins formed, and which option correctly describes the initial fertilization event?

• Identical twins (monozygotic twins) develop from a single fertilized egg. This means that one sperm fertilizes one egg, forming a zygote.
• Early in development, this single zygote then splits into two separate embryos, each developing into a genetically identical individual.

• A. two sperm cells and two egg cells: This describes the formation of fraternal twins (dizygotic twins), where two separate eggs are fertilized by two separate sperm.
• B. one sperm cell and two egg cells: This scenario is not how twins are formed.
• C. two sperm cells and one egg cell: This is a rare and usually non-viable event called polyspermy, not the mechanism for identical twin formation.
• D. one sperm cell and one egg cell: This correctly describes the initial fertilization event that leads to a single zygote, which then divides to form identical twins.

The final answer is $\boxed{\text{D. one sperm cell and one egg cell}}$