ALL BUT ONE factor is believed to be the source of Earth's internal heat energy. That is

The correct answer is the last statement, that is, continue to cool and expand.

Explanation:

The notion of expanding universe was for the first time discovered by Einstein's theory. The outcomes attained via his theory of relativity has the redshifting of light when they come from distant objects moving all through the expanding universe. Later in 1929, it was further stated by Hubble that universe is expanding when he deduced the curve between the redshift against the relative distance.  

From this it was further proved that because of the expansion of universe there can be an influence of redshift in the spectrum of light as the wavelength would have turned lengthier in comparison to the earlier that would result in the reduction in temperature related to the spectrum and hence the universe keeps on cooling spontaneously as it expands simultaneously.