Which type of workers were locked out and replaced by nonunion laborers at Carnegie's mill?

The situation referenced in your question pertains to a historical event known as the Homestead Strike of 1892, which took place at Carnegie Steel in Homestead, Pennsylvania. During this event, union workers, particularly those associated with the Amalgamated Association of Iron and Steel Workers, were locked out by the management as a response to wage disputes and bargaining negotiations.

The decision to lock out the union workers was a strategic move by the company to break the union's influence and reduce labor costs. This tactic involved hiring nonunion laborers, often referred to as "scabs," to replace the striking workers and maintain production operations. By doing so, Carnegie’s management aimed to undermine the power of the organized labor force, which was primarily composed of union members advocating for better pay and working conditions.

Thus, the correct choice is union workers, as they were the individuals directly involved in the strike and the target of management's lockout strategy. Other types of workers listed, such as contract, temporary, or seasonal workers, did not play that same pivotal role in the labor conflict at Carnegie's mill.