The correct answer is C. Apprentice.
An apprentice is a person who is learning a trade or skill from a more experienced person, usually by working with them in a practical setting. The apprentice typically works in return for being taught the trade, and may also be paid a small wage.
A disciple is a follower of a teacher or leader, and is typically someone who has been taught by the teacher and has come to believe in their teachings. A pupil is a student, and is typically someone who is learning from a teacher in a formal setting, such as a school. A sobriquet is a nickname, and is typically a humorous or affectionate name given to someone.
In the context of the question, the answer is C. Apprentice. This is because the person is working in return for being taught the trade. This is the definition of an apprentice.