Answer: (d)
Explanation:
Given
Mass of object 
Speed of object 
Mass of object at rest 
Suppose after collision, speed is v
conserving momentum

Initial kinetic energy

Final kinetic energy

So, it is clear there is decrease in kinetic energy . Thus, energy decreases and velocity becomes 2 m/s.
Answer:
a. Potential energy decreases and Kinetic energy increases
Explanation:
Because as he comes down due to its steepness the speed of the boy or you can say his KE increases and since he comes from a high position (hill) to the lower ground his potential energy decreases simultaneously