Detailed Explanation:

Saturn has the lowest density of all planets at approximately 0.687 g/cm³—less than water (1 g/cm³). Its composition is primarily hydrogen and helium gases with a small rocky core, making it extremely light relative to its volume.

Earth (5.52 g/cm³), Venus (5.24 g/cm³), and Mars (3.93 g/cm³) are terrestrial planets with dense metallic cores and rocky surfaces, resulting in significantly higher densities.

Therefore, Saturn has the least density, making Option 4 the correct answer.