Answer:
I don't know if there is any options for this question because you didn't put any, but, I think I can answer anyways.
The subject Edward and Marianne are discussing is the possibility (or not) of loving more than once in life.
Explanation:
According to Edward, Marianne's maxim is that "<em>no one can ever be in love more than once in their life</em>". He doesn't exposes his opinion about the subject (at least not in this excerpt), but seems to be more open to the possibility of falling in love more than once. On the other hand, Marianne tells him that, in fact, she didn't change her mind about it, she says that her opinions, at her "<em>time of life</em>" are already fixed. She seems to realize that she might be a little stubborn about this topic (and maybe about others too), but she thinks that probably nothing can change her thoughts.