Yeah I got the same problem, basically that's the only quest I haven't unlocked and I am wondering if it somehow got disabled in one of the newer versions? I saw some very old comments with screenshots (one below) where people had two options, when Ymir catches you messing with Christa, but in my case I have no choice and game proceeds as if I had chosen the second option (main character automatically says exactly the second line). If you somehow unlock it, please let me know. I'll also try :)
