The knights of labor allowed to join as members of their union.

They allowed all workers. For the Knights of Labor, it didn't matter to which branch of industry or manufacture or labor the worker belonged to. They accepted all people as members, regardless of their job, and tried to get better rights for all workers. This was different from how the unions worked up until then because the unions were based around the jobs, meaning that the coal union fought independently from the timber union for example. Knights of labor accepted all people and all spheres of production.

The correct answer is workers. It is because the knights of labor do not focus on which are the workers are working under or from, as long as they are citizens who are working and are normal citizens, they consider them as someone who could be considered or to be open to their union.