"Grace" rhymes with "ways": choice B. This may seem a little odd, as the two words don't exactly rhyme. This kind of imperfect rhyme is called a slant rhyme. Slant rhymes often rely on assonance (i.e., vowels that sound alike) to create the sense of rhyme. Significantly, both "ways" and "grace" are dominated by a long "a" sound, an assonance that ties the words together. Further, while the "c" in grace is softer than the "z" sound of ways, those two sounds are similar.
We know these are the words that are meant to rhyme because this is a Petrarchan sonnet. A Petrarchan, or Italian sonnet, is broken into two parts: a first part of eight lines and a second part of six lines. Barrett Browning has divided her first eight lines into two quatrains that rhyme according to the following pattern: ABBA. The word at the end of the first line, "ways," is meant to rhyme with the word at the end of the fourth line, "grace."