The best choices are B. and D.
Answer A. would not be a good explanation for the power of Bronte's vision. Answer A. implies a narrow point-of-view, which would to account for Bronte's "continuing appeal" or her powerful vision.
Similarly, Answer C. would not be a good explanation for the power of Bronte's vision. A "narrow and constrained" vision is similar to the idea found in Answer A. A narrow point-of-view would only appear to a small group of people and would not support the argument that Bronte had either a powerful vision or continuing appeal.
On the other hand, Answer B. and Answer D. indicate reasons why Bronte has both continuing appeal and powerful vision. Her work discusses questions that apply to humans in general -- not just governesses. Furthermore, the novel contains a set of multifaceted characters, who are likely to appeal to all sorts of readers.
