In one's element means comfortable and confident doing something. 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: In one's element can refer to a situation involving comfortable and confident doing something.
• Story example: A writer may use In one's element when describing a situation involving comfortable and confident doing something.
• Exam example: When a sentence contains In one's element, choose the meaning “comfortable and confident doing something.”