The correct answer is members of labor unions.
This is evident thanks to the elements of symbol in this picture.
For example, it says "steel strike." A large majority of the time, strikes by workers were organized by labor unions. Along with this, the factories shown in the picture represent the factory workers and their demands for better conditions. The people that were part of labor unions were sometimes misrepresented as being communist due to their beliefs about the government's involvement in the economy and business in general.