Although I eventually got this right, I spent a lot of time not knowing on what grounds to eliminate (B).
Is it because the passage never specifies whether the European American groups studied by these historians experienced the same weakening of community ties as the Japanese Americans studied by Fugita and O'Brien?
All we know is that these historians "unsatisfactorily" explained persistence on the basis of "peoplehood," but anything more about the groups they studied is not discussed. Is that why (B) is wrong?