Your correct answer is: B. Sentence structure and how quickly events unfold in a story
Reason? The pace is the speed of how things go, (how fast you walk, how fast a paper airplane goes by, etc.) and when writing, a slow pace is what you need. You have to make sure a story isn't rushed so you could keep the reading audience hooked. (Just make sure it's not to slow, or they'll become bored and stop reading.) Overall the reader should enjoy the pace, and should feel comfortable with it. This is what gets the reader wanting to know more information, and gets them hooked onto the events/actions that is taking place in the story. Your readers must comprehend what had happened/is happening so far before forced to understand the next part, and if the writer has accomplished this skill, they've done a great job in making your content excellent, and enjoyable to read.
Good luck with your studies, I hope this helps~!