The correct answer to the question is option 4, Earth. Density is defined as the mass of an object divided by its volume. In the context of planets, density helps us understand how compact a planet is.

Option 1, Mercury, is a small rocky planet and has a relatively high density compared to other planets in our solar system. However, it is not the planet with the highest density.

Option 2, Venus, is similar to Earth in terms of size and composition. It is often referred to as Earth`s sister planet, but it does not have the highest density.

Option 3, Jupiter, is a gas giant with a very large volume. Despite being the largest planet in our solar system, its low density makes it one of the least dense planets.

Option 4, Earth, is the correct answer. It is a rocky planet with a moderate size and density. Its dense core and mantle are composed primarily of iron and silicate rocks. Earth`s highest density makes it unique among the four options.