A stumbling block means an obstacle that makes progress difficult. 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 stumbling block can refer to an obstacle that makes progress difficult.
• Story example: A writer may use A stumbling block when describing an obstacle that makes progress difficult.
• Exam example: When a sentence contains A stumbling block, choose the meaning “an obstacle that makes progress difficult.”