The correct answer is: on.
The word “on” is used to indicate that something is covering or attached to something else. In this case, the boy is wearing a black jacket. The jacket is covering his body, so the correct word to use is “on”.
The other options are incorrect because they do not accurately describe the relationship between the boy and the black jacket. The word “with” is used to indicate that two things are together. In this case, the boy is not together with the black jacket; he is wearing it. The word “unto” is used to indicate that one thing is moving towards another thing. In this case, the boy is not moving towards the black jacket; he is wearing it. The word “in” is used to indicate that something is inside something else. In this case, the boy is not inside the black jacket; he is wearing it.