What reason best explains why metals are shiny

Metals are shiny because of their electrons. These electrons produce light when they come in contact with light. I hope this helps.

The shiny appearance of a metal is most closely related to the metal's highly mobile valence electrons. In metallic bonds, the mobile electrons surrounding the positive ions are called a(n) electron sea. As light strikes the surface of a metal, the electrons in the electron sea absorb and re-emit the light.
I hope this helps.