Supply Dropped

If you take too long to pick a reward and complete the quest after defeating the champion, he despawns, reappears near the supplies, and no longer accepts the turn-in. If/when someone else defeats him, you'll be able to turn it in.

You have reasons to suspect the Grimtotem tribe, on account of the fact that one of their champions is nearby, visibly pilfering your stuff.

I giggled. :)

As mentioned by others, if it takes you too long to decide which reward to take, the NPC will despawn and you won't be able to hand in anymore. The only solution offered so far that I have seen was that you need to wait for someone else to do the quest, and afterwards can hand in again once the NPC has been defeated again. But, what to do when nobody else is around?

Well, there is another solution!
Abandon the quest!
It will show up as the blue exclamation mark again and you can repeat the whole thing without having to wait for someone else!