He is working in return for being taught the trade.

Answer: Option C

One word substitute is Apprentice.

Disciple: a personal follower of Christ during his life, especially one of the twelve Apostles.
Pupil: a person who is taught by another, especially a schoolchild or student in relation to a teacher.
Apprentice: a person who is learning a trade from a skilled employer, having agreed to work for a fixed period at low wages.
Sobriquet: a person's nickname.