Answer:
The purpose of those words is:
D. to appeal to as large an audience as possible.
Step-by-step explanation:
If something is difficult, complicated, uncommon, you can be sure that very few people will be interested in it. Maybe it is because they cannot understand it. Perhaps it is because they think they cannot understand it and, therefore, do not even try. When Paine says he offers "simple facts, plain arguments, and common sense," he is wisely choosing such adjectives to appeal to everyone. If something is easy, simple, plain, everyone will think him/herself capable of understanding it. Everyone will be interested. And that is his purpose: to appeal to a large audience.