Explanation

The different-direction trigger word to focus on here is instead, which tells you that the type of person she is should be different from one who has a secret. Inscrutable is similar in meaning to an enigma behind the celebrated smile, so choice (E) is incorrect. Lisa de Giocondo may well have been dulcet or comely, but there is no supporting evidence for these words in the sentence. You may want to rule out choice (B) because it appears to be a trap since the sentence is about a work of art; however, an artless person is one who does not intend to deceive. A good synonym for it is choice (F), ingenuous.

Hence options B and F are correct!