It seems to me that both A and C are true.
B is false. You really can't assume that your audience knows what text you are writing about.
I usually use Grammarly which is an extension on chrome. It points out incorrect uses in commas and when you misspell words! Stuff like that!
This is a compound sentence, the comma indicates this.
It's supporting details.
Thesis and main idea go in the introduction.
