💡 Detailed Explanation
The correct answer is 「急激」 (kyūgeki), meaning 'sharp', 'rapid', or 'sudden (change)'. It is used for changes in variables or states (e.g., rise in temperature, exchange rates). 「急速」 (kyūsoku) also means 'rapid' but is typically used for progress of actions (e.g., rapid recovery). 「急進」 means 'progressive/radical'. 「強烈」 means 'intense/strong'.