Earth has retained a lot of internal heat primarily because of its (a) distance from the Sun; (b) large iron core; (c) relatively large size.

C. relatively large size

Explanation:

Earth's internal heat flow to the surface is 80% due to mantle convection while the remaining heat mostly originates in the Earth's crust and about 1% due to volcanic activity, earthquakes, and mountain building.

The earth's internal heat - stresses between the lithosphere and the upper mantle are responsible

for geological processes. These stresses are the results of motions in the upper mantle stemming from convection that push the lithosphere in different directions.

Larger planets cool slower than small planets.