Metals are good conductors of electricity because
A metal atoms are arranged in a regular lattice.
B metal ions are very close to each other.
C metal ions are free to move through the lattice.
D electrons are free to move through the lattice.
Answer is D.
Electric current is the flow of electrons. Metals are good conductors of electricity because their valence electrons are delocalised. This means that the valence electrons are not tightly associated to a single atom, but can move through the entire metal. This is the kind of bonding present between metals (metal bonds). In a metal bond, electrostatic forces are at play between the delocalized electrons and the positive metal ions. This forms a 'sea of electrons'. Since the electrons are free to move in the lattice of positive metal ions, they can easily move.
This, however, is not the sole requirement. They also need partially filled energy bands. When both are satisfied, application of an electric field will cause the to move across the lattice and migrate.
Hence, metals are good conductors because (D)electrons are free to move in the lattice.