Answer:
a. in supernovae and star collisions
Step-by-step explanation:
The periodical table contains some heavier elements, which are formed as neutron stars pairs hit eachother and erupt cataclysmically.
The star emitts very large quantities of energy and neutrons during supernova, which allow for the production of heavier elements than iron, such as uranium and gold. All these elements are ejected into space during the supernova explosion.