A necessary evil means something unpleasant but needed. 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 necessary evil can refer to a situation involving something unpleasant but needed.
• Story example: A writer may use A necessary evil when describing a situation involving something unpleasant but needed.
• Exam example: When a sentence contains A necessary evil, choose the meaning “something unpleasant but needed.”