Explanation:
Here mechanical energy is conserved.
Kinetic energy + Potential energy = Constant.
First the child does 5.0 J of work on a spring while loading a ball into a spring-loaded toy gun.
   That is potential energy = 5 J
   Kinetic energy = 0 J
   Kinetic energy + Potential energy = 0 + 5 = 5 J Â
Secondly the spring is released.
   That is potential energy = 0 J
   Kinetic energy = ?
   Kinetic energy + Potential energy = Constant = 5 J
   Kinetic energy + 0  = 5 J
   Kinetic energy is 5 J
Option D is the correct answer.