To make a bigger splash, the student should drop the same ball from a greater height. This option is not listed among the provided responses, but it is the correct approach for maximizing the splash upon impact with the water.
Among the options given, the best alternative would be to use a ball with the same diameter but more mass. A heavier ball will have greater momentum when it hits the water, potentially resulting in a larger splash.
Therefore, the best choice would be: They can use a ball with the same diameter but more mass.