A bone of contention means a subject that people argue about. 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 bone of contention can refer to a subject that people argue about.
• Story example: A writer may use A bone of contention when describing a subject that people argue about.
• Exam example: When a sentence contains A bone of contention, choose the meaning “a subject that people argue about.”