Answer: option C. the presence of free-flowing ions.
Explanation:
Ionic compounds dissociate in water forming solutions with free ions. The solution conducts electricity and is named electrolyte.
The ions are species with charge: cations are ions with positive charge and anions are ions with negative charge.
The ions become the carrier of the charge so they are the responsible for the flow of current.
So as the electrons are the carriers of the charge in the solid conductors (copper, aluminun, and others), the ions are the carriers of the charge in solutions.That is how the the electrolyte solutons conduct electricity.