semelis: The answer at B15 is wrong. Even if the player wanted sente so badly as to give up 10 points, B16 would have been better.
In these "best endgame" problems maybe both players' moves should be left to the reader, to find the best sequence for both colours.
If there's an interesting tesuji hiding in a suboptimal sequence, make a new problem from the mistake onwards.
posetcay: It is about finding the tesuji, it is not necessary for both players to play the optimal endgame moves, only the strongest resistance. B18-A18 would be the optimal endgame but that sequence doesn't make sense in a tsumego.
jhubert: B15 is said correct but in this case, sacrifice 2 stones let the opponent save the corner stones
Ivan Detkov: It is interesting. It seems like it is always possible to save the corner by sacrificing the two stones. It could be both branches are good because one is sente and another is gote.
The answer at B15 is wrong. Even if the player wanted sente so badly as to give up 10 points, B16 would have been better. In these "best endgame" problems maybe both players' moves should be left to the reader, to find the best sequence for both colours. If there's an interesting tesuji hiding in a suboptimal sequence, make a new problem from the mistake onwards.