Final answer:
The least dense layer of the Earth is the crust, with densities ranging from 2.6 g/cm³ for continental crust to 3.0 g/cm³ for oceanic crust, which is significantly lower than the mantle and core densities. Option C.
Step-by-step explanation:
Among the layers of the Earth mentioned, the crust is the least dense. The Earth's crust can be subdivided into two types: continental and oceanic crust, both of which are less dense than the mantle and core layers below them. The crust has an average density of around 2.6-2.7 g/cm³ for continental crust and 2.9-3.0 g/cm³ for oceanic crust.
The mantle, which contains the asthenosphere and the upper mantle, shows an increase in density downward, with the upper mantle's density starting at about 3.5 g/cm³. By contrast, the outer core and inner core consist largely of iron and nickel, with the outer core being liquid and the inner core being solid due to immense pressures. The densities of these layers are significantly higher than that of the crust, therefore making the crust the correct answer to the student's question regarding the least dense layer. Option C.