The correct option is
D Praise
The word 'denigrate' refers to criticizing someone or something unfairly or disparaging.
Option A, belittle refers to dismissing as unimportant.
Option B, believe refers to accepting that something is true, especially without proof.
Option C, doubt refers to a feeling of uncertainty or lack of conviction.
Option D, praise refers to expressing warm approval or admiration of. Thus, according to the meaning of the given word, this will be the correct antonym to 'denigrate'.