The correct answer is option A. Jem and Scout know that Atticus is serious about the above statement because that was the only time they heard Atticus say it was a sin to do something. Atticus is very honest about what Jem and Scout can and cannot shoot at.
I believe the answer is A, "that was the only time they heard Atticus say it was a sin to do something" because, by process of elimination, he specifically was a good shot, and didn't tell his kids to shoot anything in particular. Therefore the only logical conclusion in my eyes is A. Have a nice day! :)
