Outer core composition and thickness?

• A. A layer of molten silica
• B. Mass of molten iron about 2200 km thick that surrounds the solid inner core
• C. A solid iron sphere
• D. A gaseous state

Answer: (B)

The outer core is a liquid layer of metallic iron (with some nickel) that surrounds the solid inner core, and it is about 2,200 kilometers thick. This description matches the idea of a molten iron mass enveloping the solid inner core, which is the correct characterization of the outer core.

Why this fits best: molten iron at high temperatures and pressures allows convection, which drives the geodynamo that generates Earth's magnetic field. The other ideas don’t fit because the outer core is not made of silica, not solid iron, and not a gas—it's a dense, molten iron alloy layer surrounding the inner core.