Metals are good conductors of electricity due to the presence of delocalized valence electrons. These electrons are not bound to any particular atom and are free to move throughout the metal lattice. When an electric field is applied, these electrons can move freely and carry the electrical current.

The other options are incorrect because:

a) Covalent bonding: Covalent bonding is a type of chemical bonding in which electrons are shared between atoms. This type of bonding is not responsible for the electrical conductivity of metals.

b) They are shiny and hard: The shiny and hard nature of metals is due to their closely packed atomic structures. This does not directly contribute to their electrical conductivity.

c) Ionic bonding: Ionic bonding is a type of chemical bonding in which electrons are transferred from one atom to another. This type of bonding is not responsible for the electrical conductivity of metals.