Premise: Nuclear power plant are far more expensive to build than conventional plants
Conclusion: Safety considerations aside, nuclear power plants are not economically feasible
Flaw: Takes for granted that the cost of building a plant is a significant factor in determining its economic feasibility.
A potential answer choice will attenuate this gap.
(A) Safety consideration have no effect on the premise-conclusion relationship here. We're specifically told that apart from safety considerations, nuclear power plants are more expensive than conventional plants. So as far as this particular argument is concerned we need to disregard safety considerations. Consequently, the fact that safety considerations can increase or decrease costs and or economic feasibility has no effect on the reasoning.
(B) Irrelevant. The fact that conventional plants spend more time out of service relative to nuclear power plants does not make it more likely that the latter are economically infeasible. Even if we were to assume that this would render conventional plants economically infeasible, the reasoning would remain unaffected.
(C) This looks good. It doesn't match the flaw we identified but that's okay. This provides us with an additional reason to believe that nuclear power plants are not economically feasible. If they are more expensive to build than conventional plants and also have a shorter life span than conventional plants, it is more likely that nuclear power plants are not economically feasible. This doesn't make the argument perfect by any means but it does make it a bit stronger.
(D) The current of construction relative to the old cost has no effect on the reasoning. The fact that nuclear power plants cost less to build today does not make more or less likely that nuclear power plants are currently economically infeasible. All it does is suggest that in the past, nuclear power plants were even more economically infeasible than they are today.
(E) This has no bearing on the argument. While the cost of building conventional plants relative to nuclear plants is used to justify the conclusion, this is completely irrelevant. The fact that conventional power plants may become economically infeasible does not make it more or less likely that nuclear power plants are economically infeasible.
(C) is clearly the best answer, pick it and move on.