Question

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

Explanation:

Ionic compounds have strong ionic bonds. High melting and boiling points (so low ones are wrong). They are hard (due to lattice structure) but brittle (layers shift and repel). In solid, ions fixed - can't conduct (but when molten/solution, they can, so inability as building material? No, as building material is solid. Many ionic compounds dissolve (like NaCl), so inability is wrong. Hardness (structural support) and brittleness (can be shaped/cut in some ways) make them useful as building materials.

Answer:

hardness and brittleness