The answer is D.
Step-by-step explanation:
It can't be A, because it would be very hard to keep the amount of trees being cut down and the amount of trees being planted the same. Also, the entire problem would not be taken care of in a few months because people would have to constantly keep it up.
It can't be B, because it is a concern anywhere in the world about cutting down forests. If everyone would go somewhere else to find wood, then the forests in those areas would disappear also.
It can't be C, because populations in the rest of the world are not falling. The need for wood in those places is also rising.
It is D, because the forests have been cut down at very fast rate. Our forests will disappear if this continues.
Hope this helps! :)