In one's prime means at the best or strongest stage. 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: In one's prime can refer to a situation involving at the best or strongest stage.
• Story example: A writer may use In one's prime when describing a situation involving at the best or strongest stage.
• Exam example: When a sentence contains In one's prime, choose the meaning “at the best or strongest stage.”