The correct option is D doubtful
The word 'definite' refers to something clearly stated or decided.
Option A, doubtful refers to feeling uncertain about something.
Option B, regrettable refers to giving rise to regret.
Option C, certain refers to something that is unquestionable or sure.
Option D, unpredictable refers to something that is incalculable.
Thus, according to the meaning, the word 'doubtful' is the opposite of the word 'definite'.