**Core Concept**: Prefix rules (e.g., 'ir-' before 'r'). **Answer Analysis**: - **C. Irregular** is correct. The prefix 'ir-' is used before roots starting with 'r' (e.g., regular → irregular). - **A, B, D** use incorrect prefixes ('in-', 'im-', 'il-'), which do not align with phonetic rules. **Common Mistake**: Misapplying prefix variations (e.g., using 'il-' instead of 'ir-').