The best answer is letter B. Usually in films, fade ins/fade outs are used to show a passage of time in between scenes to avoid mixing one scene to another. Fades typically last for a second or two which is equivalent between24-48 frames but in a narrative, it depends on the director on how he or she would like the mood of the scenes to be like.
Fades are used to differentiate a scene that is playing and the next scene that will be played. Without the fades, viewers will see 2 different scenarios as one and that would mess up their understanding of what the film wants to convey its viewers. That is why it is up to the director’s decision of the length of the fade ins and fade outs depending on the story.