Which of the following has the greatest electrical conductivity? B(s) CaCl2(l) Fe2O3(s) Mg(s)?

Substances can conduct electricity either by delocalized electrons or by mobile ions.
B(s) cannot conduct electricity because it is a non-metal which does not have delocalized electrons or mobile ions to conduct electricity.
CaCl₂(l) can conduct electricity. It is the molten liquid of an anion compound which has mobile Ca²⁺ and Cl⁻ ions to conduct electricity.
Fe₂O₃ is an ionic solid. The Fe³⁺ and O²⁻ ions are well arranged. It cannot conduct electricity because its ions are not mobile.
Mg(s) is a metal which can conduct electricity by delocalized electrons.

Delocalized electrons have higher efficiency in conduction of electricity than mobile ions, because delocalized electrons have much small size and much higher mobility. Therefore, Mg(s) has greater electrical conductivity than CaCl₂(l).

The answer: Mg(s)