At the mercy of means completely controlled by someone or something. 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: At the mercy of can refer to a situation involving completely controlled by someone or something.
• Story example: A writer may use At the mercy of when describing a situation involving completely controlled by someone or something.
• Exam example: When a sentence contains At the mercy of, choose the meaning “completely controlled by someone or something.”