This question seems to be based on your particular textbook and so you should look there to be sure that you get the answer that your teacher expects. That said, the answer has to be B, in my opinion.
B has to be part of the answer because so many of the environmental problems we face are caused at least partly by increasing population. It is the increasing population (and its increasing use of energy even on a per capita basis) that causes, for example, global warming. So C alone cannot be the only answer.
We are left then, to choose between B and D. I do not believe that our problems are insoluble and therefore I would say B is the correct answer.