Edward: "perhaps, then, you would bestow it as a reward on that person who wrote the ablest defence of your favourite maxim, that no one can ever be in love more than once in their life—your opinion on that point is unchanged, i presume? " marianne: "undoubtedly. at my time of life opinions are tolerably fixed. it is not likely that i should now see or hear any thing to change them."
the subject edward and marianne are discussing is

marriage
persuasion
second attachment
elinor's romantic life

The correct answer is second attachment.

Explanation:

Edward and Marianne had previously discussed the topic of being able to fall in love more than once in a lifetime, and Marianne had expressed that she believed there was no such a thing. In this conversation, Edward alludes to her statement: "(...) your favourite maxim, that no one can ever be in love more than once in their life". She insists that this is still her opinion, since at her age is is unlikely she will change her views on anything.

the answer is:

magna carta