This line comes from “She Walks in Beauty”, line 7 and this connotes balance.
At the start of the poem, it already introduces two opposite components - darkness and light. In this line, the two opposites refer to her dark hair and her lightened face. So the first choice is wrong since it only speaks of the woman's hair. In this line, it is more about the balance of the two opposing forces. Even if she weren't proportioned just right, she would still be half beautiful.
