The theme of the poem is the girl's beauty, internal as well as external. The 1st stanza emphasizes her physical beauty. The 2nd and 3rd stanzas emphasize both her physical and spiritual, or intellectual, beauty. However if I had to choose from the 4 options it would be b. inner beauty since during the whole poem the girl is pensive and thoughtful apart from also showing facial expressions from many wandering ideas or thoughts.
I also base my answer on this part of the poem: “The smiles that win, the tints that glow, / But tell of days in goodness spent.” By the end of the poem, the poet declares that the girl's beauty is the reason for her good character, saying that this outer beauty leads to inner beauty and innocence.