This quest is available starting Chapter 4.
Go to the Arena. Talk to the comedian sitting on a bench north of the Arena. He will ask you if you're interested in bananas. Say yes (the left option). He will also ask you to get him a banana. Buy or give him a banana and he will give you 50 Prank Bananas
Reward: 50 prank bananas
You can repeat this quest as much as you want.