The Crust and Lithosphere (which are almost synonymous) are not typically divided into two distinctive layers, so we can reject them.
The correct answer is is the core: (b). The inner core is solid and it's made of iron and nickel. The outer layer is liquid and this is the reason for the differentiation of the two parts. <span />
The bulk density of a planet will reflect the relative amounts of the materials the world is made of, regardless of what materials occur at the surface.
