Jslate:
the variation where you sacrifice 4 stones but it is still counted right should probably be counted incorrect, and there should be a Hint that there is a better variation. just my opinion

Neal Wright:
Black D2, White C3, Black C2 should be correct. Now D1 and B3 are miai. If White plays D1, Black plays B3 and if White connects at E4, Black gets a second eye at D3. If White connects at D3, Black captures 4 stones at E4. I got this wrong on a golden tsumego using that solution, but I think mine should be correct.

Djab:
One combination is not optimal but it works (so it is "correct") : Q18 R17 R18 and then Q19 and S17 are MIAI

GoDave89:
Die Bestrafung für Q17 müsste nur R17 sein - Q18 ist falsch