The correct option is C simple
The word 'sophisticated' refers to involving a great deal of worldly experience, especially in the field of fashion and culture.
Option A, vigorous refers to being strong, healthy and full of energy.
Option B, simple refers to something easily understood, or done.
Option C, artificial refers to something not existing naturally.
Option D, superficial refers to existing or occurring at or on the surface.
Thus, according to the meaning, the word 'simple' is the opposite of the word 'sophisticated'.