Granted that the second solution uses miai to make the false eye, but this “incorrect” solution is arguably better, as it kills the whole corner.
Since this is not a killing excercise, and it is beginner level after all, I don’t think the second one should be the only solution, but probably both should be included as correct (after follow-up).
Good catch, and it shows that it would be helpful if we could play more moves in these problems (like in every good Tsumego app or page), so as to try different variations.
BUT maybe this specific problem here could be solved if it the wording was changed:
Black to play. Make a white eye false in one move.
I did most of the tutorials with my son recently, and IMHO the general pattern is there are lots of “correct” answers in the tutorials that are bad moves (sometimes a little bad, sometimes unambiguously bad) whose merit is that they demonstrate mastery of the relevant concept (like, “capture the marked stone” or “make the marked point a false eye”). I can certainly see why problems where the “correct” answer involves failing to kill a dead group are potentially misleading, but I suspect it will be difficult to find a bright-line rule for the OGS tutorials, because so many are arranged around demonstrating understanding of a concept, rather than finding the best move.
On the other hand, in general I agree with the principle that free and open source tsumego collections should have a single correct opening move. While John Fairbairn and others have argued at length that this hasn’t traditionally been a requirement in pro-edited tsumego books, we see here the practical difficulty of curating problems with multiple valid solutions.
Still better if finding the best move is part of the deal even if it’s not explicitly asked.
If there is a superior move, but not really so direct and simple as the answer given as correct, then this may trouble the reader even if he doesn’t find it.
So I tried to construct a position where C could be better with a ladder, but even here B (e5) is better because:
white can resist g5 peep at H6 to keep the ladder good whilst also keeping the fight good if black pulls out f4.
if C (f5) instead then g5 is still a ladder breaker that threatens to pull out the e4 cutting stone and this one there’s no good counter to both keep ladder good and cutting stone suppressed.
