This weakens the argument. It suggests that the reduced risk of serious injury since 1955 is actually due to technological innovation, and not to the legislation introduced in 1955.

This doesn't weaken the argument. Perhaps the government regulated in such a way that reduced the chance of carelessness leading to injury. If that was the case, then the author’s argument remains intact.

This doesn't weaken the argument, because we're not interested in the annual number of injuries overall. The argument is focused on high-risk industries, not all workplaces. Even if C reflected some increase in the annual number of injuries in high-risk industries, that wouldn't tell us anything about the overall safety of individual workers. Maybe the number of workers in these industries has drastically increased, so even though they are each individually safer, the overall number of injuries per year has gone up.

Irrelevant. The argument is only about high-risk industries, not non-high-risk industries.

This doesn't weaken the argument. For all we know, workplace safety conditions have improved in all industries because the government began regulating across all industries. We're looking to undermine the idea that government legislation specifically targeting the high-risk industries improved worker safety in those industries. E doesn't give us a reason to doubt that claim.