A one-track mind means a mind focused on only one thing. 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 one-track mind can refer to a mind focused on only one thing.
• Story example: A writer may use A one-track mind when describing a mind focused on only one thing.
• Exam example: When a sentence contains A one-track mind, choose the meaning “a mind focused on only one thing.”