Cool one's heels means wait for a long time. This answer is correct because the expression has a fixed figurative meaning in English. In an SSC idiom question, students should match the whole phrase with its accepted meaning rather than translate each word separately.

• Classroom example: Cool one's heels can refer to a situation involving wait for a long time.
• Story example: A writer may use Cool one's heels when describing a situation involving wait for a long time.
• Exam example: When a sentence contains Cool one's heels, choose the meaning “wait for a long time.”