Choose the option that means the opposite of the given word- Capricious 

• Words that are opposite to each other  in meaning are referred to as antonyms.
• The meaning of the word capricious is unaccountable or sudden shift of behaviour and mood.
• We have known the meaning of the capricious, let us find the antonym of the capricious.
• The meaning of the option D is dutifully firm or resolutely. So the antonym of the capricious is steadfast.
• Option A, B and C are the incorrect answers as they are not the antonyms of the capricious.

Hence, option D is the correct answer.