The correct option is B promote
The word 'thwart' refers to prevent someone from accomplishing something.
Option A, favour refers to approval, support or liking from someone.
Option B, sustain refers to strengthen and support mentally or physically.
Option C, promote refers to furthering the progress of something.
Option D, frustrate refers to preventing from progressing, succeeding.
Thus, according to the meaning, the word 'promote' is the opposite of the word 'thwart'.