Explanation
Correct answer: A.
Choice A is the best answer. "Stimulated" means "encouraged interest or increased activity in." Because the newspapers discussed the benefits of westward migration and "inspired readers to relocate," we can infer that they encouraged people to move west.
Why the other choices are wrong
Choice B
Choice B is incorrect. "Assigned" means "gave a job or duty." Newspapers do not have the power to assign people to move west, although they can encourage it.
Choice C
Choice C is incorrect. "Opposed" means "disapproved of." We can tell that this isn’t the case, because the newspapers discussed "the easy fortunes and high salaries miners could make in California" and "inspired readers to relocate."
Choice D
Choice D is incorrect. "Disregarded" means "ignored." If the newspapers are writing about the west and "inspir[ing] readers to relocate," they can’t be ignoring it at the same time.