Kinetic energy depends on the mass of the object and its velocity when it hits the water. When all objects are dropped from the same height, they fall under the influence of gravity and will have the same velocity just before impact if we neglect air resistance.
The formula for kinetic energy is:
\[ KE = \frac{1}{2}mv^2 \]
where \( m \) is mass and \( v \) is velocity.
Since all objects are dropped from the same height and have the same velocity just before hitting the water, the object with the greatest mass will have the greatest kinetic energy.
From the table:
- Table tennis ball: 2.7 grams
- Baseball: 147.0 grams
- Small rock: 248.2 grams
- Penny: 2.5 grams
The small rock has the greatest mass (248.2 grams), so it will have the greatest kinetic energy when it hits the water.
The answer is small rock.