Because if your writing a story you want to be focused on the characters, the setting, and most importantly the plot! Without these what is the structure of a story??
The answer would be B. Setting, Character and plot. Those three things are what make up a story and if you're a writer you need these three things to help hook the readers into your story.
Videos are NOT always a primary source for information. This is because anyone can make a video and post it online, but not everyone is an expert. A primary source should usually be from an expert in that field.