Modifiers have to accompany the thing they are modifying or go as close to it as possible. If this does not apply, then a misplaced modifier occurs. Therefore, your answer would be B, that is, the sentence contains a misplaced modifier.
The sentence makes it sound as if the speaker's arm was sliding down the hill. As arms cannot walk, it is possible to say that the modifier has been misplaced. The explanation would be that the speaker was sliding down the hill when he got his arm broken.
