which types of stars are thought to be the source for enriching the universe with heavy elements? Explain why

The production of elements heavier than Iron takes place by adding neutrons to the atomic nuclei. These neutral particles do not feel any electrical repulsion from the charged nuclei. They can therefore easily approach them and thereby create heavier nuclei. This is indeed the way the heaviest chemical elements are built up.

There are actually two different stellar environments where this process of "neutron capture" can happen.