Can a baby sit on the lap of an adult in the backseat of a car with seat belts?

Depends on where you are talking about, and if the car is moving or not.

In the United states, all infants must be in child safety seats when the car is moving. If you stop to nurse the child in the backseat, then its ok but not while you are driving.

The differences between the states usually are in what size or age the child has to be in order to not have a special seat.