A dead letter means a rule, law, or plan that is no longer effective. 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 dead letter can refer to a rule, law, or plan that is no longer effective.
• Story example: A writer may use A dead letter when describing a rule, law, or plan that is no longer effective.
• Exam example: When a sentence contains A dead letter, choose the meaning “a rule, law, or plan that is no longer effective.”