For your first question about why nativists opposed immigrants, the most fitting response would be:
They felt immigrants were loyal to their home countries.
Nativists often believed that immigrants had divided loyalties and that their allegiance was to their nations of origin rather than to the United States.
For your second question regarding the goal of implementing a poll tax, the most accurate response is:
To stop African Americans from voting.
Poll taxes were primarily used as a means of disenfranchising African American voters and, in some cases, poor white voters, effectively limiting their participation in the electoral process.