I think it is b. Most beautiful, as c and d don't make sense. A is a possibility, but I'm pretty sure it is b. Most beautiful.
In My Last Duchess, the painting and the art represent something that is long gone, most noticeably, his love for art. He claims that he once loved art but now that love is gone and he hates it as much as it hates him.
Identify and develop your topic .
- hope this helps :)