I can agree with that, to an extent.
I actually felt Ending D was the happiest of them all. But even Ending D isn't happy:
No matter your actions, nothing has changed. Everyone is still going to die, or in the true human's case,
go insane, anyway. Yonah still has her illness, it doesn't go away magically. Ending "E," which was written in the
book and someone posted a link in this thread would probably be considered the happiest, and true ending simply
because the cycle ends.