Answer:
In the given line, "And the blood of her veins in the moonlight throbbed to her love's refrain," it's most likely that "her love's refrain" is a poetic way of expressing the passionate and emotional response she feels for her loved one. It's not specifically referring to any of the options you provided, but rather, it's a figurative description of her intense emotions.
You should recheck the entirety of whatever your reading and if you still can't find the answer it'd be a good idea to email your professor.