At the back of one's mind means remembered but not thought about all the time. 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: At the back of one's mind can refer to a situation involving remembered but not thought about all the time.
• Story example: A writer may use At the back of one's mind when describing a situation involving remembered but not thought about all the time.
• Exam example: When a sentence contains At the back of one's mind, choose the meaning “remembered but not thought about all the time.”