Choice D is an excessively convoluted way of saying 'jobs die with people not returning instead of other people replacing them'. The core of the argument is basically the following:
Since more people would live --> net earnings will increase --> result in greater GNP and more tax.
We need to connect more people living and earnings increasing. In the real world, more people surviving does not necessarily lead to greater earnings, since they may simply be replaced. If they do get replaced, then (economically speaking) whether they return or not won't affect net level of earnings. This is where (D) is getting at. Without this assumption of no-replacement, this argument falls apart.
This world of no replacement is the 'base' for the comparison 'employment increased'. Thus, when more people live through ST, they would return to their jobs, which would have been eliminated had they not returned. As a result, earnings are increased.
Consider the following illustration:
In this world of non-replacement, the number of jobs is at 100.
An accident occurs in which 50 people are involved who are all employed. They all die. Then, because jobs disappear with these people not returning, the number of job is reduced to 50.
Under the same scenario, let us assume that STs are built and that it allows 10 people to live and return to their work. In this 'non-replacement with ST' world, the number of jobs is 60.
Therefore, 'if more people lived-->higher employment'.
I don't think LSAC expects students to do this much mental gymnastics within the short time period given; rather, a more sane approach would be eliminating choices with negation technique.
A. per capita income comparison. Negated, "per capita incomes are different". So...?
B. At present, no STs exist. Negated, "ST's exist in X". So what.
C. St centers should not be more expensive than treatment elsewhere. Negated, STs are more expensive. We should be able to rule this out conclusively because the scope of the argument is treating injuries that 'ONLY SPECIALIZED TREATMENT CENTERS can provide'. So, comparison of costs is useless
D. Negated, it says "more lives saved --> no increase in employment'. If we substitute employment with 'sources of income,' (which is interchangeable in my opinion under acceptable LSAT assumptions) then, which is pretty relevant.
E. Negated, it says most people in X who are seriously injured in car accidents receive specialized treatment.
So...? What the argument advocates is a 'system'. Individual existence of such centers or treatment is not of concern.