Answer:
d. the mobility of electrons
Step-by-step explanation:
Covalent bond is the bond which is formed with the sharing of the electrons between the two atoms which are taking place in the bond. It is generally formed between the atoms with similar electronegativity values.
Metallic bond is the bond which is formed by electrostatic attraction force between the positively charged metal ions and the conduction electrons.
Thus, in metallic bond, there is a formation of conduction band and the valence band where the electrons have mobility and have transitions in between them but in covalent bonding, the electrons are shared in the bond and are not mobile.