After St. John repeatedly pressures Jane to marry him and go to India, Jane is tempted to accept St. John's marriage proposal. Why does this information precede the climax?

Although I think there is some credibility to answer B, I think D is probably the answer your teacher wants.  On the one hand, if we go with answer B, we are noting that Jane understands that she wants more from a marriage than her relationship with St. John would provide.  She does not just want to be a sort of side-note in his work.  D helps Jane realize how she truly feels about Mr. Rochester and how she needs to see if he still feels the same way, even after all that has happened.  The climax is that point in the story when the end becomes the end, so it makes sense that her realization leads to them coming together again and being together again.