Final answer:
The missing material for testing conductivity in ionic compounds is the solid state. Ionic compounds do not conduct electricity in solid form but do when dissolved in water or molten due to the free movement of ions. Correct answer is b) Solid state.
Step-by-step explanation:
The missing material for testing conductivity in ionic compounds is: b) Solid state. Ionic compounds in their solid state do not conduct electricity because the ions are fixed in place within the crystalline lattice and cannot move freely to conduct an electrical current.
However, when these compounds are dissolved in water or are in a molten state, the ions are free to move, and thus, the substance can conduct electricity. This is confirmed by the fact that a water solution of an ionic compound conducts electricity well, as ions are the charge carriers that facilitate the flow of current in an aqueous solution. So, the Correct answer is b) Solid state.