Looking at the answer's options, we could discard some of them, like A, B, and D, because it would be counterproductive since the reason of not knowing things about sexual education could in fact increase the human population, or the fact of eliminating land use on agriculture in each country, for example, surely would generate a deficit in these resources supply for each country, or the fact of using only non-renewable resources would result in the increasing contamination of the air and water, for example.
So, having this clear, we could say that the statement which describes a better solution to the overpopulation problem, would be the C option, as you said, "birth regulations are imposed in some countries to reduce population numbers.