Question

The element in a fluorescent lightbulb that absorbs UV light and releases visible light energy is ____?

Answer 1

You're answer will be a phosphor 
The element in a florescent lightbulb that absorbs UV light and releases visible light energy is --a phosphor--. 

Answer 2

So in a fluorescent light bulb the mercury atoms release the UV light. This UV light is absorbed by the phosphorous powder coating inside and this releases the visible light.
So your answer is phosphorous powder.