As long as all the waves stay in the same medium, the intensity of
any waves ... electromagnetic or mechanical ... decrease in proportion
to the square of the distance.
If the distance increases to 3 x the original distance, then the intensity
changes to 1/3² or 1/9 of the original intensity.
I suppose choice-'d' is the correct one, but I have to tell you that
the phrase "nine times as low" is mathematically meaningless,
and it really grinds my gears.
Answer:
The distance to the moon greater than the distance across the Atlantic ocean by an order of 1.
Explanation:
We have the average distance across the Atlantic ocean, 
the average distance to the moon, 
The factor by which the distance to moon is greater than the width of Atlantic ocean:



i.e.
the distance to the moon greater than the distance across the Atlantic ocean by an order of 1.
Well because it has momentum and it needs to be slowed down.