Final answer:
The inner core is the densest layer of Earth, primarily composed of iron and nickel under high pressure.
Step-by-step explanation:
The layer of Earth with the highest density is the inner core. The Earth's interior is composed of distinct layers with varying chemical compositions and mechanical properties. The inner core lies at the very center and is thought to be composed of an iron-nickel alloy with possibly some amounts of sulfur, all compressed to a very high density due to the extreme pressures. Based on seismic studies, the inner core is solid and has a higher density compared to the liquid outer core, the silicate mantle, and the crust. The density of the mantle increases downward, but it is the inner core that remains the densest because of its metallic composition and the pressure exerted upon it.