The answer is: A
Explanation
The correct option is A: Upon his birth.
According to Section 20 of the Transfer of Property Act, 1882, when an interest in property is created for the benefit of an unborn person, he acquires a vested interest in the property as soon as he is born, unless a contrary intention appears from the terms of the transfer. This means that the unborn person has a right to the property from the moment of his birth, even if he may not be able to enjoy it immediately.
For example, if A transfers his property to B for life, and after B's death to C's unborn son, then C's son will acquire a vested interest in the property as soon as he is born, but he will have to wait until B dies to get the possession of the property.