To increase the density of a hollow lead ball filled with water, the most effective method among the options you provided is to remove the water and melt the ball into a solid sphere.
Here's why:
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Removing the water: Water has a relatively low density compared to lead. By removing it, the overall density of the system increases since you would only have the lead mass, which is denser than water.
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Melt the ball into a solid sphere: Solid lead is denser than a hollow structure filled with water. By melting the hollow lead ball into a solid sphere, you eliminate the air gap inside, and the density of the lead increases, which enhances the overall density of the object compared to a hollow structure.
Increasing the size of the ball and adding more water would not effectively increase the density since the added water doesn't add as much weight compared to the increase in volume, thereby lowering the overall density.
In summary, to increase the density of the hollow lead ball filled with water, the correct approach is to remove the water and melt the lead into a solid sphere.