In metallic solid, the forces of attraction is between the mobile valence electrons and the

The question is asking us to identify the particle that the mobile valence electrons in a metallic solid are attracted to. In a metallic solid, the valence electrons are not tightly bound to any particular atom, but rather are free to move throughout the entire solid. These electrons form a "sea" of negatively charged particles that is dispersed throughout the solid. The positive charges in a metallic solid are located in the nuclei of the atoms that make up the solid. Since opposite charges attract each other, the mobile valence electrons are attracted to these positively charged nuclei. Therefore, the correct answer is: "positively charged nuclei".