Final answer:
The true statement about the electron sea model is that the valence electrons are mobile and flow freely between metal nuclei, which characterizes the nature of metallic bonding.
Step-by-step explanation:
The statement that is true of the electron sea model is: a. The electrons are mobile and are free to flow between metal nuclei. This model is used to describe metallic bonding, where the structure of a metallic bond is characterized by the presence of delocalized valence electrons that are able to move freely throughout the metallic crystal. These electrons, often denoted as a 'sea of electrons,' are not exclusively associated with any one metal cation but move about, providing a glue-like effect that holds the metallic structure together.
The positive metal nuclei and the sea of electrons combine to form a lattice-like structure, which is very different from that seen in covalent and ionic bonds. Therefore, while the valence electrons do contribute to the bonding, they are not specifically being 'shared' in the same way as in a covalent bond, making options b and c incorrect. Additionally, option d is a broad statement about metallic bonding rather than a specific feature of the electron sea model.