Final answer:
The correct answer is that when the iron nuclei in the core of an evolved high-mass star start to come apart, they explode in a supernova.
Step-by-step explanation:
When the iron nuclei in the core of an evolved high-mass star start to come apart, the star cannot produce energy through fusion of iron, as it requires rather than releases energy. As the core contracts due to gravity and heats up to extreme temperatures, it collapses until its density exceeds that of an atomic nucleus, forming a neutron star. The core's collapse and rebound transfer energy outward, resulting in the explosive ejection of the star's outer layers in a catastrophic event known as a supernova. It is this type II supernova that disperses heavy elements beyond iron into space and not the formation of a black hole, helium nuclei, or a simple collapse into a neutron star without the accompanying explosion.