Crystals that have a sea of delocalized electrons
a. are made of alternating ions.
b. are very brittle.
c. have covalent bonds.
d. conduct electricity.

D. conduct electricity.

Explanation:

Delocalized electrons are electrons that are not associated with any atom or ions. These electrons differ from localized electrons which associates with a particle. Localized electrons for example are the ones transferred between ions when ionic bonds form and are shared when covalent bonding occurs.

The sea of delocalized electrons found in crystals helps in electric conduction in most metals we have. The electrons are free and mobile and not attached to any atom. This makes them readily available for electrical conduction.

D. conduct electricity

Explanation : Crystals are usually metals and the atoms of these metals form metallic bond in which the valence electrons gets delocalized and moves through the entire. The atom becomes positively charged when it loses its electron and the interaction between the electron and the positively charged atom is called as the metal bond.

Movement of electron or delocalized electron along the solid can conduct electricity. More the number of free electrons, greater will be the conductivity.  

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.