Red-eyed dust bunny
I'm going to object on the grounds that it's wrongWryfool, come on down!
The real answer is infinite HD. While 6 HD is the highest that can be affected by the save or die effect, but those above that HD total still take 1d10 points of damage/round. Since the question asked for the max HD affected by the spell, the question's clearly wrong. I suspect the authors of the quiz forgot that, because the 1d10 damage/round is a new addition to the spell. In first edition, 7 HD creatures could stand in a cloudkill all day and take no damage.
(Not that any of us got the real correct answer.)