The answer is D.
"Keep", "factor", and "calculate" are all verbs in the list of things the new system allows teachers to do. The other sentences are inconsistent with their list of verbs/adjectives/nouns. Thus, option D is the only one that is grammatically correct.
I am not too good at explaining, so let me know if you are still confused.
Hope this helps! :)
Plot : What is the sequence of events in the story?
Characters: What do we learn about the boy whom the story is about?
Point of View: Is the boy telling the story or is an outside narrator telling the story?
Theme: What does the author want us to learn from the story?
Setting: Where does the boy live?
The way the author makes the story is what actually affects all of that