Identical twins form from the fertilization of:

Identical twins, also known as monozygotic twins, originate from the fertilization and subsequent division of a single fertilized egg cell. This single egg is fertilized by a single sperm, creating a zygote. After fertilization, the zygote undergoes cleavage and divides into two separate embryos. This means that both embryos share the same genetic material, which is why they are identical.

In contrast, options involving two egg cells or two sperm cells would lead to fraternal twins, which arise from the fertilization of two separate egg cells by two separate sperm cells, resulting in siblings that share about 50% of their genetic material. The notion of separated embryos does not apply to identifying how identical twins are formed since the process begins with a single fertilized egg rather than two distinct embryos. Thus, the correct understanding of how identical twins are formed directly relates to the fertilization of one egg by one sperm, which leads to the development of two genetically identical individuals.