Answer:
Everyone was replaceable; it an unskilled worker was fired for asking for fair wages, another worker was ready to take the job immediately
Explanation: Here's why it can't be any of the others:
A: "Workers were not smart enough to stick up for themselves or ask for better working conditions" There were people who asked for better working conditions but were fired.
B: "Unskilled workers did not deserve protection at work until they learned the skills of the workplace." They never got protection at all yet.
C: "Working conditions were not bad, there were always workers willing to take the job." The working conditions were awfully dangerous.