Earth's Internal Heat Engine
For this question, Wikipedia is probably your best recourse. 

To summarize, the heat in the earth's core comes from two places: it is left over from the formation of the earth and it is still being generated by radioactive decay of isotopes that were trapped in the earth as it formed. 

The radiogenic heat is still being added to the earth through decay of long lasting isotopes. When the earth formed, there were probably many significant heat generating isotopes, but now after 4.5 billion years, it is pretty much just Thorium-232, Uranium-238, Potassium-40, and Uranium-235.

So there are multiple answers to this question in the sense that there are the two sources listed above, but otherwise, no, this is the answer.

This question has some more information related to heat transport to the surface that you may find relevant.