At what speed does the kg ball move ?

 Choice A) 2 meters per second

Explanation:

The smaller ball has momentum of

p = m*v

p = (1 kg)*(4 m/s)

p = 4 kg*m/s

All of this momentum transfers into the larger ball because the smaller ball comes to a complete stop.

For the larger ball, we have p = 4 and m = 2. Let's find v.

p = m*v

4 = 2*v

4/2 = v

2 = v

v = 2 m/s which is why the answer is choice A

The larger ball moves at a speed of 2 meters per second. The speed is cut in half compared to the smaller ball because the larger ball has more inertia (aka more mass), and therefore it takes more energy to move it. If you apply the same energy to each, then the smaller object moves faster.