Before schools were commonly used and accepted as educational institutions if you wanted to learn a trade, you would need to find a master and to ask him to be his apprentice. If the master accepted you, then he would transmit his knowledge and technique to you. Thus the right answer is He is an apprentice training to learn a trade.
A good example is shown in the picture because you can see the master an the apprentice, but you can also find this situation in some movies, when a young guy moves in with a blacksmith and learns from him before becoming a master himself.
