A narrow margin means a very small difference. 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 narrow margin can refer to a very small difference.
• Story example: A writer may use A narrow margin when describing a very small difference.
• Exam example: When a sentence contains A narrow margin, choose the meaning “a very small difference.”