Chapter 5 - Work and Energy - Learning Path Questions and Exercises - Conceptual Questions - Page 173: 8

To decrease the Kinetic energy of an object as much as we can, we must reduce the speed by half.

Since, Kinetic energy =$\frac{1}{2}mv^{2}$ If we reduce mass by half, the Kinetic energy will be $\frac{1}{2}$ of initial Kinetic energy. If we reduce speed by half, the Kinetic energy will be $\frac{1}{4}$ of initial Kinetic energy. So, to decrease the Kinetic energy as much as we can, we should reduce the speed to half.