Option 1 -> Passionate is a synonym of ardent, meaning showing strong emotions or enthusiasm.
Option 2 -> Fervent is also a synonym of ardent, meaning having intense feeling or enthusiasm.
Option 3 -> Indifferent means showing no interest or concern, which is the opposite of being ardent.
Option 4 -> Vague means unclear or indefinite, which doesn't directly oppose the emotional intensity of ardent.
Hence, Option 3: Indifferent -> "Ardent" describes someone who is passionate and enthusiastic, while "indifferent" describes someone who lacks interest or concern, making them true antonyms -> correct