[duo797]

I got the same answers:
E,F,H and I were the only ones to go to the concert, but 'I' didn't really enjoy the concert.

Chryuop: C is the past, potential, negative. C is saying 'I couldn't go'.

I'm gonna take a guess at the last person and say it is 行っておければよかった
If that is what the last person is saying, I'm not sure what the ておく form is supposed to mean here.