All in a day's work means a normal part of one's duty or job. 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: All in a day's work can refer to a normal part of one's duty or job.
• Story example: A writer may use All in a day's work when describing a normal part of one's duty or job.
• Exam example: When a sentence contains All in a day's work, choose the meaning “a normal part of one's duty or job.”