Final answer:
The layer of Earth with the greatest density is the Inner core. It is composed of an iron and nickel alloy, solid due to the extreme pressures, and has a very high density compared to other layers of the Earth such as the mantle and the crust. Option D is correct.
Step-by-step explanation:
The layer of Earth with the greatest density is the D. Inner core. The Earth's inner core is composed primarily of an iron and nickel alloy and is both incredibly dense and under tremendous pressure. It is solid, despite the extreme temperatures, due to this immense pressure.
Surrounding the inner core is the outer core, which is liquid and contributes to the generation of Earth's magnetic field. The inner core's density is the result of the heavy compositions of iron, nickel, and possibly sulfur, all being compressed under intense gravitational forces.
In contrast, the mantle, which includes both the upper and lower parts, is denser than the crust but less dense than the core. It increases in density from about 3.5 g/cm³ to more than 5 g/cm³ from the upper to the lower mantle. However, the inner core is denser still, making it the correct answer to the question posed.