Step 1: Definition of "Ardent"
The word ardent signifies intense passion, eagerness, or enthusiasm. It denotes strong emotional investment and zeal.
Step 2: Evaluate each choice
- (a) Indifferent: means lacking interest or concern; apathetic. This is the opposite of passionate. ✔
- (b) Zealous: means enthusiastic and devoted. This is a synonym, not an antonym. ✘
- (c) Fervent: means showing intense passion, a synonym of ardent. ✘
- (d) Enthusiastic: means showing keen interest. This is also a synonym. ✘
Step 3: Conclusion
The only antonym for Ardent is Indifferent.
\[\boxed{\text{Answer = (a) Indifferent}}\]