Answer: Under the given circumstances , the following would be a positive externality for the global environment: Fewer people move from rural to urban areas, reducing urban sprawl.
Urban sprawl can lead to several negative externalities such as : pollution, exploitation of natural resources at an exponential rate, unemployment etc.
Therefore, if fewer people move from rural to urban areas, this will reduce urban sprawl and further every other negative externality associated with it.
The correct option is (A)