Metallic bonding is responsible for the unique properties of metals, such as their malleability, ductility, and conductivity. It occurs due to the attraction between the positively charged atomic nuclei and the delocalized cloud of electrons surrounding them. In metallic bonding, the valence electrons are not held tightly by any one atom but are free to move throughout the metal lattice. This allows for the sharing of electrons between many atoms, creating a cohesive and stable structure. Therefore, the correct answer is "Attraction between the atomic nuclei and cloud of electrons."