Which answer best completes the sentence? of all the dancers in the ballet, giselle was by far the . a. most gracefullest b. more graceful c. the gracefullest d. most graceful

When you use "by far" in one running sentence, you don't necessarily have to use most in the same sentence. People would argue between C. And D.
If I had to pick D. Then the sentence would look like "Giselle was by far the most graceful. C. Giselle was by the far the Gracefullest, but that word itself is incorrect. SO it has to be D. Most graceful.