Which two layers of earth are mainly composed of metallic elements such as nickel and iron, which are abundant in meteoroids and comets? inner core outer core lower mantle upper mantle crust

The correct answer is the inner core and outer core.  

Similarly to other terrestrial planets like Venus, Mercury, and Mars, the Earth comprises many layers. This is the outcome of it going through planetary differentiation, where the denser substances sink to the middle to form the core, while the lighter substances get produced around the outside. The core, that is, the outer core and the inner core comprises mainly of nickel and iron, while the upper layers of the Earth comprise minerals and silicate rock.