Viviparity refers to the reproductive strategy in which

Viviparity is the way of reproducing in which the young one grows inside the mother’s body and is fed by her (in mammals through the placenta) until it is born alive. Humans, goats, cows, dogs and whales are viviparous.

So the correct description is that the offspring develop and are nourished inside the female’s body. Animals whose young develop outside the mother, inside an egg with its own food store, are oviparous — like birds, most fish, frogs and lizards.

The two answers about fertilisation are not definitions of viviparity. External fertilisation happens outside the body, as in frogs and most fish, and can never be viviparity. Internal fertilisation is needed for viviparity, but it is not the same thing — birds and snakes that lay eggs also fertilise internally. Viviparity is about where the young develops and is fed, not just where the sperm meets the egg.