All in the name of science.
How far the marshmallow travels will depend on how far you pull the elastic band back and therefore how much force you apply to the arm of the catapult.
So, the ‘effort’ you put into pulling back the elastic band will propel the load (marshmallow) through the air.
Meanwhile, air resistance will be acting on the load as an opposing force and this will slow it down.
Gravity will be acting on the marshmallow too, trying to pull it down towards the ground.