Final answer:
The major contributing factor for the Earth's high internal temperature is the heat from the very hot Earth's core, which is primarily due to primordial heat and radioactive decay.
Step-by-step explanation:
The Earth's high internal temperature is primarily due to heat retained from the planet's formation and heat produced by the decay of radioactive isotopes. Among the options provided, A. The Earth's core is very hot is the correct answer. The core is the innermost part of the Earth, consisting of a solid inner core and a molten outer core, which is responsible for the geothermal gradient observed on the planet. This heat from the core is transferred through the mantle to the Earth's surface via convection and conduction.
Additionally, it's worth noting that while the mantle, crust, and atmosphere do have varying temperatures, they are not major contributing factors to the core's heat. Instead, their temperatures result from the heat that originates in the core and is transferred outward.