I believe that the speaker in the ballad of Robin Hood and the Scotsman is the option 'C' - Someone who knows the story.
If you read the ballad, it can be concluded that Robin Hood is on his own. Friar Tuck was not with him. This ballad is also written in the past tense so Robin Hood could not have been the narrator. One can infer that the story was of the fight was either told to the narrator of this ballad by Robin Hood himself or the Scotsman.
