I'm not a verbal expert but I'll try my best, please correct me if If my reasoning is flawed
Conclusion is "the higher rate of injuries is undoubtedly due to a higher accident rate for inexperienced workers", if we can somehow state that experienced workers are also experiencing injuries then we can undermine this statement.
(A) Many of the inexperienced workers hired when the firms' work loads increase are hired only for temporary positions.
(out of scope)
(B) The studies of workplace safety were focused only on injuries that resulted in lost workdays.
(out of scop)
(C) There is a much higher rate of injury in construction firms than in manufacturing firms.
(we are talking about both construction and manufacturing, so doesn't make any sense to compare these two to undermine conclusion)
(D) The accident rate for experienced workers tends to increase whenever the firms' work loads increase.
(correct, clearly states that there is some other reason for high accident rates)(E) Firms that hire inexperienced workers for potentially dangerous jobs are required to provide them with training
(what if even after the training these the accident rate is still high in these inexperienced workers? it doesn't do anything to the conclusion)
Kudos help!