The correct option is A Calm
An antonym is a word that is opposite to the meaning of another.
The given word 'furious' means to be extremely angry about something, for example, 'He was furious when he found out the truth.'.
From the options, 'calm' refers to a pleasant mood, not showing any anger or extreme emotions; it is opposite in meaning to the given word. (Consider the statement: He was calm when he found out the truth.) Hence option A is correct.
'Crazy' means extremely enthusiastic, which isn't the opposite of 'furious', hence option B is incorrect.
'Angry' means 'a feeling of strong displeasure or hostility', it is synonymous with the given word. (A synonym is a word that is similar in meaning to another.)
Hence option C is incorrect.
'Violent' means being aggressive or fierce, it does not mean the opposite of 'furious', hence D is also incorrect.