The answer is A. who and whom are personal pronouns
Well if the characters in the story know about an apocalypse then they should prepare for what could happen. they should use the knowledge of what they know about the apocalypse, they should use that knowledge and prepare.
The process of identifying word roots has more to do with stripping away additional word elements than directly locating the root.
For example, if you had the word "hardly" you'd strip away the suffix "ly" and have the root remaining, "hard".
Hope that helps!