A dark horse means a person who is unexpectedly successful or has hidden ability. 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 dark horse can refer to a person who is unexpectedly successful or has hidden ability.
• Story example: A writer may use A dark horse when describing a person who is unexpectedly successful or has hidden ability.
• Exam example: When a sentence contains A dark horse, choose the meaning “a person who is unexpectedly successful or has hidden ability.”