The correct answer is D. A prepositional phrase means that it starts with a preposition. And in these examples, D is the only one that starts with a preposition "on".
To start off, you want to get zero population growth. That means you can eliminate A and D automatically since it's the opposite effect of what you want.
That leaves B and C. Both are good idea if I may say but, they are unreasonable and unethical. (Not as much as A and D) Although, it is only just a question.
After a good time of thinking this through I would pick letter C. Let me explain though.
It is very easy to increase the death rate. There are so many ways to achieve zero population growth by doing so.
Now, obviously this means you would have to (for a nicer term) put down people. Those I will not list but, you can surely image them if you'd wish.
The reason I took B out of the equation is because, I feel as though it would take too long. That's the only reason.
Like I said I would pick C. I hope this helps!!
