A sitting duck means an easy target for attack or criticism. 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: A sitting duck can refer to an easy target for attack or criticism.
• Story example: A writer may use A sitting duck when describing an easy target for attack or criticism.
• Exam example: When a sentence contains A sitting duck, choose the meaning “an easy target for attack or criticism.”