Final answer:
The item described is Guppy's Tail, which increases the chance of finding chests but decreases the chance of finding consumables.
Step-by-step explanation:
The item described in the question is Guppy's Tail. When picked up, Guppy's Tail increases the player's chance of finding chests at the end of a room, but it also decreases the chance of finding consumables like keys and bombs.
This item is one of the three required items to transform into Guppy, a powerful and unique character in the game. Collecting all three Guppy items grants the player with powerful abilities and transformations.
So, the correct answer is A) Guppy's Tail.