Nativists were often hostile to immigrants for several reasons, including concerns about cultural differences, competition for jobs, and the fear that immigrants would not assimilate into American society. However, from the provided responses, the more fitting reason is:
They were concerned that immigrants were too different from "Americans" and threatened the social order and identity of the nation.
This viewpoint reflects a fear of cultural dilution and economic competition, rather than direct resentment toward labor unions.