The kinetic energy of an object just before it hits the ground can be calculated using the formula for gravitational potential energy, which is converted to kinetic energy as the object falls:
\[ KE = mgh \]
where:
- \( KE \) is the kinetic energy,
- \( m \) is the mass of the object,
- \( g \) is the acceleration due to gravity (approximately 9.81 m/s²),
- \( h \) is the height from which the object is dropped.
From this formula, we can see that the kinetic energy is directly proportional to the height. Therefore, the higher the drop height, the more gravitational potential energy is converted into kinetic energy, resulting in greater kinetic energy upon impact.
Among the given options, the highest height is 40 cm. Thus, dropping the object from 40 cm will result in the most kinetic energy when it hits the ground.
So, the answer is 40 cm.