Question

why is the earths core so hot?
particles in the iron and nickel are packed tightly into a solid state as the weight of the world presses down
the core absorbs heat through thermal transfer.
it is made of radioactive material that burns at high temperatures.
nuclear fission takes place at the core.

Explanation:

The Earth's core is hot mainly due to radioactive decay of elements like uranium, thorium etc. Radioactive materials release energy as they decay, contributing to the high temperature of the core. Thermal transfer (like conduction, convection) is a way heat moves but not the source of core's heat. Nuclear fission (splitting of heavy nuclei) doesn't occur in the Earth's core in the way it does in man - made reactors. The packing of iron and nickel particles (while related to core's structure) is not the cause of its high temperature.

Answer:

It is made of radioactive material that burns at high temperatures.