The correct option is D congruous
The word 'grotesque' refers to being comically or repulsively ugly or distorted.
Option A, good refers to being desired or approved of.
Option B, decent refers to being appropriate and fitting.
Option C, filthy refers to being disgustingly dirty.
Option D, congruous refers to being in agreement or harmony.
Thus, according to the meaning, the word 'congruous' is the opposite of the word 'grotesque'.