The core has the largest density, being made of iron and nickel
The mantle has an intermediate density and the crust has the lowest density.
I believe it is the outer crust but i will do more reaserch for you :)
Your answer will be 4, our magnetic field.
Earth itself is largely protected from the solar wind by its magnetic field. However, some of the charged particles are trapped in the Van Allen radiation belt.
Hope this helps, have a great day! :)