"4,000,000,000 J" is the "Potential Energy" of water dropping over "Niagara falls".
Step-by-step explanation:
The body possess some energy according to the position of the object that is the when an object is placed at a height it stores some energy it is called "Potential Energy".
Formula used to calculate the "Potential Energy" is

Where, "m" is mass of an object given that 8,000,000 kg, "g" is acceleration due to gravity is
and "h" is height of the object is 50 m.
Substitute the given values in the formula,
Potential energy =

Potential energy = 4,000,000,000 J