Which characteristic(s) makes ionic compounds useful as a building material?

low melting and boiling points
hardness and brittleness
inability to conduct electricity
inability to dissolve in water

The correct answer to the question is hardness and brittleness.

EXPLANATION;

Before coming into any conclusion, first we have to understand the formation of an ionic compound.

An ionic compound is generally formed between electropositive metal and electronegative nonmetal.

The electropositive metal will lose electrons and forms a cation . The electronegative non metal will accept electrons and forms anion. The cation and anion so formed will attract each other due to the coulombic force of attraction which results into the formation of an ionic compound.

Due to the strong electrostatic force of attraction between ions, the melting and boiling points of ionic compounds are very high. The ionic compounds have very hardness, and are brittle in nature due to this property of the ionic compound.

As water is a polar compound, so  ionic compounds are dissolved in water. Ionic compounds are neutral in nature, but they conduct electricity when they are dissolved in water.

As ionic compounds have hardness, and are brittle in  nature, it can be considered as the building material.