Step 1: Concept
Benign means gentle, kind, or not harmful (often used in medical contexts).
Step 2: Meaning
Antonym means the word with the opposite meaning.
Step 3: Analysis
- Malignant = very infectious or harmful (the opposite of benign).
- Friendly/Gentle/Mild = synonyms of benign.
Step 4: Conclusion
Hence, the correct antonym is malignant.
Final Answer: (C)