Concept:Metallic bonding involves attraction between free-moving valence electrons and positively charged metal centres.
Explanation:In a metallic solid, each atom releases its valence electrons.
These electrons move freely through the whole metallic structure.
The remaining part of each atom is a positively charged nucleus with its inner electrons.
The mobile electrons are attracted to these positively charged nuclei.
This electrostatic attraction holds the metal lattice together.
So the force of attraction is between the free electrons and the positively charged nuclei, not atoms, neutrons, or negative ions.
Answer:D. positively charged nuclei