Which of the following could be the identity of a white crystalline solid that exhibits the following properties?

• It melts at 320$^\circ$C
• It does not conduct electricity as a solid.
• It conducts electricity in an aqueous solution.

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Going through the answer choices:

Glucose, $\ce{C6H12O6}$, does not dissociate in solution. It does not form ions and therefore does not conduct electricity in any phase.

$\ce{NaOH}$ will likely form an ionic solid. In ionic solids, the ions fill the lattice points and are unable to move. Ionic solids have a high melting point due to the attraction between charged ions. In solution, $\ce{NaOH}$ will dissociate into ions and therefore conduct electricity.

$\ce{SiO2}$ will form a network atomic solid. Each silicon atom is connected to four oxygen atoms, forming a large network structure. The melting point is extremely high due to these bonds. $\ce{SiO2}$ does not conduct electricity since it does not dissociate in water.

Copper has a high melting point as a metal. It does conduct electricity as a solid due to highly mobile free electrons.