What action did McCormick take against strikers prior to the Haymarket Riot?

The correct choice reflects that McCormick employed a lockout strategy against the strikers. A lockout is a tactic used by employers where they temporarily close the workplace to prevent employees from working, effectively denying them wages and employment, often in response to strikes or labor disputes. This action intensifies the confrontation between strikers and management and can escalate tensions, which was evident leading up to the Haymarket Riot. The lockout served as a way for McCormick to demonstrate his refusal to negotiate under pressure and to maintain control over the labor situation at his company.

The other options involve more cooperative or conciliatory actions that would not align with the hostile environment created by the lockout. Negotiating a new contract or incorporating workers' demands would imply a willingness to engage with the labor movement in a positive manner, which did not occur. Informing them of job availability does not fit the scenario either, as it suggests an approach that would seek to offer employment rather than escalate a conflict. Thus, the tactical choice of a lockout is consistent with the labor relations dynamics of that period and the events that followed.