heron:
I think C18 should be accepted as a solution. The eternal life solution is worse under superko rulesets.

TherapieGo:
I agree with caranthir, the rule says "The same situation cannot occur twice in a game", so when White takes on E19, Black should not be allowed to take on D19 as we have already seen this situation. The right and only answer to this problem should therefore be to start with C18 and launch the ko.

posivb:
[w (defender), upper-left] Sequence "A18-C18-C17-B19-G19..." seems wrong. If b play F19 instead of B19, w is dead.

caranthir:
At least with the sequence given in the solution, w (defender) would not live under any superko rule - neither positional nor situational superko - ie. AGA, NZ, or even (strictly interpreted) Chinese rules. White would not be allowed to recapture with D19 right after b E19 (that would recreate earlier board position, also with the same player to move), and would have to play some non-local threat first (I can't find any local ones). Anyway, after using a threat, b could just play F19 (and likewise over and over again), so w could not even live in ko.

Hataycan:
It should say white to live.