Even though you didn't give us the fragment with the comma slice, it isn't really necessary here as we can answer just based on these 4 options here. All options apart from the second one are correct - only the second option doesn't fix the mistake of the comma splice, whereas the other options do but connecting the sentences properly.
Answer:
The protagonist is the main character, often a hero.
The antagonist is the character who opposes the protagonist, often a villain.
Explanation:
The protagonist is Donovan and the antagonist is Donovan's friends from "Hard Castle Middle School" .
A. To explain how meaning is sometimes lost when communicating only through e-mails
