Ok, I found it in that book of yours. The rule there is about Future Indefinite and Be Going To while making predictions: predictions with evidence - be going to, predictions without evidence Future Indefinite.
The only context we have is a picture of apparently a news reporter with a background screen with another reporte and a writing "King arrives". I personally would consider it as prediction with evidence, thus B would be correct, the only thing is that the picture doesn't indicate any time while in B it's written 6 o'clock, so maybe not. Sometimes drawings in grammar books don't quite spur the associations the authors hope for.