First time using the forums so hope I’m posting in the right category 
I came across this problem…but the answer seems a bit strange, doesn’t it? I think they just put the diagrams in the wrong order, as the explanation of diagram 8 makes sense with diagram 9 picture, and vice versa:
But my friend thinks the actual answer it’s diagram 9 picture (the diagonal move)
What do you think? 
Agree. Diagrams were swapped. The one with option C should be Diagram 9
I can also confirm from recalling some online teaching lectures from pros that the textbook-correct peep is the knight’s move in this shape, not the diagonal, which is consistent with the diagrams being swapped.
Swapped. May you tell us where the mistake was, could interest more people (and even the writer could correct)?
The diagrams are indeed swapped. However, the text in the Diagram 9 paragraph about “attempt of White to defend the cutting point in sente through exchanging White A and Black B” being bad is wrong rather than a typo, though was standard theory I also learnt pre-AI. In a similar (but not same) position in the British championship a few years ago my opponent solidly connected and it was the biggest mistake of the game so far (-27%, see move 35 of Uberdude's journal • Life In 19x19): he should have played the “crude” pushing-in move, even though it does allow the 2nd line sente peep (or choice of trade), in order to get sente because that was more important. I checked this position with shallow KataGo and it’s true here too.
