I think it's the
Let me first clarify a possible loophole in the typical answer:
It could be disputed that "eating" is the process of inputing food in mouth, probably chewing, and then swallowing. There is an alternative: IV. So the lions could be alive without eating by having IVs on them.
If this is a case,
Assasins are professionals and would generally not attack someone without reason.
It sounds expensive for someone to gather many assasins in one room, full of guns, just to see the answer someone would give to a riddle. If this is NOT so, it means the assasins are there involuntarily. You would probably "free" them and have a bunch of armed assasins on your side to a common enemy...the bastard that plays games with people's lives. (Where's Minsc?)
On the other hand,
It would be also expensive to being able to maintain a bunch of lions' IVs over 3 years.