Question

what kept early earths surface molten for millions of years?

a kinetic energy from asteroid and planetesimal impacts being converted to heat; friction
b heat from the sun being much closer to earth
c radioactive decay in the atmosphere
d volcanic eruptions from the core

Explanation:

Early Earth's molten surface was sustained by the heat generated when the kinetic energy of frequent asteroid and planetesimal impacts converted to thermal energy, along with frictional heating from these collisions. The Sun was not closer enough to cause this sustained melting; radioactive decay occurred in the interior (not atmosphere); volcanic eruptions were an effect, not a cause of the molten surface.

Answer:

a. Kinetic energy from asteroid and planetesimal impacts being converted to heat; friction