Force = (mass) x (acceleration)
Force = (18 kg) x (3 m/s²) = 54 newtons
As long as you continue pushing the cart with 54 newtons of force,
it will accelerate at 3 m/s².
At the instant you release it, or keep your hands on it but stop pushing,
it will stop accelerating. It'll continue forward at the speed it had when
the 54 newtons of force stopped.
Answer:
System D --> System C --> System A --> System B
Explanation:
The gravitational force between two masses m1, m2 separated by a distance r is given by:

where G is the gravitational constant. Let's apply this formula to each case now to calculate the relative force for each system:
System A has masses m and m separated by a distance r:

system B has masses m and 2m separated by a distance 2r:

system C has masses 2m and 3m separated by a distance 2r:

system D has masses 4m and 5m separated by a distance 3r:

Now, by looking at the 4 different forces, we can rank them from the greatest to the smallest force, and we find:
System D --> System C --> System A --> System B