Explanation:
Full Explanation:

Identical twins, also known as monozygotic twins, occur when a single fertilized egg (ovum) splits and develops into two embryos. Here’s how it happens:

- Step 1: One ovum is fertilized by a single sperm.
- Step 2: The fertilized egg (zygote) divides into two separate embryos.
- These two embryos will have identical genetic material because they came from the same fertilized egg, hence they are genetically identical.


Let’s break down the other options:

#B. an ovum fertilized by two sperms and then divides into two, each forming an embryo
- This scenario would lead to non-identical twins, called fraternal twins or dizygotic twins, as each embryo would have a different genetic makeup. Two sperms fertilizing one ovum is a rare occurrence and not a typical pathway for identical twins.

#C. two ova are fertilized at the same time each forming an embryo
- This describes the process of fraternal twins (dizygotic), where two separate eggs are fertilized by two different sperms, resulting in non-identical twins.

#D. two ova are fertilized by a single sperm each forming an embryo after fertilization
- This option does not accurately describe any twin formation process. Each ovum needs to be fertilized by its own sperm, and this would not result in identical twins.

#E. one ovum first divides into two and each half is fertilized by a sperm before developing into an embryo
- This is an incorrect explanation. Fertilization typically occurs before the division of the ovum. Splitting occurs after fertilization, not after separate fertilizations.



Summary:
Identical twins occur when one ovum is fertilized by one sperm and then splits into two, forming two embryos that are genetically identical.