The conclusion that can be drawn is that Nativists opposed i migration from Asian nations. This can be seen from the stereotypical Asian features of the person being punched, and how a similar person is in a place named “China Town”.
Here are why the other reasons are incorrect:
American citizens never were targets of discrimination during this time, unless you count immigrants who became citizens (but not white American nationals), but this is not the answer and the cartoon does not illustrate Americans being discriminated against.
Though Americans opposed immigration from some European nations, the political cartoon is targeting Asian immigrants.
Finally, nativists did not focus on native Americans (mainly immigrants) and this cartoon did not target Native Americans. However, it’s likely that nativists at the time disliked native Americans. (But the cartoon isn’t about that).
Hope this helps!