heron: Why is there another move after E12? I can only guess moves randomly from there, very frustrating.
caranthir: D9 isn't quite random but follows from normal shape intuition. If black were to cut at E11, we should atari at D11 and now D9 is in an exceptionally good place. All that said, another good move isntead of D9 would be to play D11 in order to emphasize the corner/side. Yet another one is to play E7; this applies pressure to the two enemy stones in the center (cutting this would lead to yet another profitable squeeze). Katago likes these moves too.
***
Anyway, taking into account some earlier comments too, I changed the problem so that it stops at E12.
John: I believe this problem is wrong. The variations should stop after B E12, this is the sequence provided in Kano Yoshinori's book (problem 5). I have no idea why G10H11E12G13E11 is marked as incorrect and what is the 'correct' last move.
TomaLexi: The last move should be allowing a lot more options, including moves like E11 and E7/E8.
Ivan Detkov: The tesuji is standard and it works well but not in this position. This task should be present on 19x19 board with a couple more stones on it. Unfortunately, I do not know how to increase the size of the board.
D9 isn't quite random but follows from normal shape intuition. If black were to cut at E11, we should atari at D11 and now D9 is in an exceptionally good place. All that said, another good move isntead of D9 would be to play D11 in order to emphasize the corner/side. Yet another one is to play E7; this applies pressure to the two enemy stones in the center (cutting this would lead to yet another profitable squeeze). Katago likes these moves too. *** Anyway, taking into account some earlier comments too, I changed the problem so that it stops at E12.