Answer: medium of exchange.
Explanation:
Bart is able to change the $5 bill he has, for 50 water balloons. This means that the 5 dollar bill has a given value, and that is may be used to, in this case, obtain water balloons. This means that the 5 dollar bill is a "medium of exchange" which means "A widely accepted token that goods and services"
So this is an example of money as a medium of exchange.
