Assertion: Twins may arise from a single egg or from two eggs.
Reason: One egg gives rise to identical twins by separation of blastomeres and two eggs produce non-identical twins

The correct option is B Both assertion and reason are true and the reason is the correct explanation of assertion.

Two offspring produced from the same pregnancy are twins. There are two types of twins: Identical and non-identical or fraternal twins.

When a single-celled zygote formed by the union of a sperm and an egg divides into two blastomeres which separate and become two embryos, which are genetically identical, share all the genes and have the same sex, they will result in monozygotic or identical twins.

In contrast, fraternal twins or non-identical twins are formed from the fertilization of two eggs by two separate sperms. The resultant embryos are dizygotic, not genetically identical, and share half of their genes (like any other normal siblings) and sex can be either same in both or different. This type of twinning occurs mostly in the case of assisted reproductive procedures. The two developing babies have separate placenta.