Question

nuclei heavier than iron are formed in pulsars. in the cores of main sequence stars. in the explosions of massive stars. in the accretion disks of black holes.

Explanation:

Main - sequence stars fuse lighter elements up to iron. Pulsars are not the main site for forming heavy - nuclei. Accretion disks of black holes are mainly about matter being pulled in and radiating energy, not forming heavy nuclei. In the explosions of massive stars (supernovae), the extreme conditions allow for the formation of nuclei heavier than iron through processes like the r - process.

Answer:

in the explosions of massive stars.