Final answer:
The Haymarket Square incident's significance is that it stirred up fear of anarchism and linked it to the labor movement, leading to a backlash against unions and labor activism option (c).
Step-by-step explanation:
The significance of the Haymarket Square incident in 1886 was that c) It stimulated a hysterical wave of fear of anarchism and its alleged connection with unionism.
The incident, which happened during a protest for the eight-hour workday, turned violent when a bomb was thrown into the crowd, resulting in the death of a police officer and injuries to others.
In the aftermath, anarchists were arrested and four were executed, leading to a public association between radicalism, specifically anarchism, and the entire labor movement.
This association led to a decline in union membership and an increase in public fear and suspicion of labor activism.
The Haymarket affair had far-reaching effects on the labor movement in the United States, contributing to the perception of labor activism as dangerous and radical.
While the organizers were not responsible for the violence, the media's portrayal and the subsequent trial cemented the tragic events of May 4 as a turning point in labor history, influencing public opinion and labor policies for years to come.