Final answer:
Jupiter is denser than Saturn primarily because its greater mass leads to a more compressed interior than Saturn's, resulting in a higher density.
Step-by-step explanation:
The main reason why Jupiter is denser than Saturn is due to its greater mass, which leads to higher compression of its materials. Despite both Jupiter and Saturn being predominantly composed of hydrogen and helium, Jupiter's additional mass significantly compresses its interior, much more so than Saturn's. This results in a higher overall density for Jupiter. To clarify, option (d) 'The extra mass of Jupiter compresses its interior to a greater extent than that of Saturn' is the correct answer to why Jupiter is denser than Saturn.