Why Is It That Terrestrial Planets Are Thought to Have

Why is it that terrestrial planets are thought to have dense iron cores?

A) Thermonuclear reactions produced iron as a fusion product in the earlier phases of planetary evolution (thermonuclear accretion) .
B) During the formation of the planets, heavy elements accumulated first, followed by the accretion of lighter elements onto them (bimodal accretion) .
C) The planets were molten early in their life, and the heavy elements sank and lighter materials floated to the surface (planetary differentiation) .
D) Magnetism in the iron was sufficiently powerful to pull iron atoms to the center as the planet formed (magnetic sorting) .