Dropping is most commonly used as accidentally letting something fall out of your hand.
Landing is most commonly used to describe airplanes coming back to ground.
Falling means something is moving downwards uncontrolled. e.g. When you drop something it is falling.
Descending is also most commonly used with airplanes, it means to go to a lower altitude. Another situation where you might hear it is hiking, e.g. "Descending from the mountain."
Hope this helps!