The correct option is B profound
The word 'superficial' refers to something not through, deep or complete.
Option A, profound refers to something very great or intense.
Option B, difficult refers to something needing much effort and skill to accomplish.
Option C, secretive refers to concealing feelings and intentions.
Option D, mystical refers to relating to supernatural or paranormal.
Thus, according to the meaning, the word 'profound' is the opposite of the word 'difficult'.