A sea change means a very big change. 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 sea change can refer to a very big change.
• Story example: A writer may use A sea change when describing a very big change.
• Exam example: When a sentence contains A sea change, choose the meaning “a very big change.”