posetcay:
Ok here B2 is the only move that results in a direct ko, B1 results in a thousand-year ko and C1 results in a multi-step ko, both of which are inferior. Should we change the answer to B2 only or should any kind of ko be acceptable here since it is an intermediate collection?

Lyde:
Top left orientation: For B19, the defendent can play C19 (continuation: B18, E19, C18, Tenuki.) This probably makes B19 inferior to the other solutions because it takes an extra step to start the KO. In any case the current variation B19, B18, C19, E19 is already KO at that point (The attacker doesn't need to play locally anymore). Also, for C19 the answer should be B19 because: C19, B18, B19, A19 is already KO again.