Question

which ionic compound is used as a building material?
salt
limestone
magnesium oxide
iron oxide

Explanation:

• Salt (NaCl) is primarily used for seasoning/de-icing, not building.
• Limestone (calcium carbonate, $\text{CaCO}_3$, an ionic compound) is widely used in construction as a building stone, aggregate, or to make cement.
• Magnesium oxide is used in refractory materials, not general building.
• Iron oxide is a pigment or rust, not a building material.

Answer:

limestone