The conclusion is that, for economical reasons, country X should institute a transportation system for getting people to specialized trauma centers. According to the author, it makes economical sense to do this because it would save the lives of many people, and this in turn would result in both an increase in the gross national product, as well as an increase in government revenues from the taxes paid on these earnings.
I recommend first looking for a flaw, rather than looking specifically for a necessary assumption. The flaw here is that even if the transportation system saves the lives of many people, this doesn’t necessarily mean that more money will be made. Maybe country X has a crappy economy and saving lives would result in even higher levels of unemployment and poverty. So before looking at the answer choices I would prephrase the correct answer along the following lines: The argument depends on the assumption that if more lives are saved, more money will be made as a result. As it turns out (as is often the case on a necessary assumption question) none of the answer choices look exactly like my prephrase, but the prephrase helps direct us towards answer choice D. And notice that if D were false- that is, if more persons survived serious injury there would NOT be a net increase in employement- then the argument falls apart.