The conclusion is that "if major industries increase their capital reserves, the employment rate will not decline in the future." Why? Because major industry did not have capital reserves. The author assumes that having capital reserves is sufficient to prevent a decline in the employment rate. We are asked to cast doubt (i.e., weaken) the author's claim.

(A) Whether the drop in employment was foreseen does not relate to the core of the argument, which is that capital reserves will prevent another decline in the employment rate.
(B) The fact that some major industries had appreciable capital reserves does not contradict the claim that an increase in these reserves would prevent a future drop in employment rates.
(C) CORRECT. The author neglects to take into account the fact that other factors, such as an increase in labor costs, could adversely affect the employment rate. For example, if the cost of labor becomes prohibitively expensive, even with increased reserves, the employment rate could decline.
(D) Legislation mandating a certain level of reserves does not contradict the claim that increased reserves would prevent a drop in employment rates.
(E) The fact that the employment rate was more severe this year than last does not contradict the claim that an increase in reserves would prevent a drop in the employment rate.