A grain of truth means a small amount of truth. 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 grain of truth can refer to a small amount of truth.
• Story example: A writer may use A grain of truth when describing a small amount of truth.
• Exam example: When a sentence contains A grain of truth, choose the meaning “a small amount of truth.”