Which one of the following metal oxides is antiferromagnetic in nature?

Choose the correct option for crystalline solids based on the following statements:
$(1)$ They do not have a sharp melting point.
$(2)$ They gradually soften upon heating.
$(3)$ They have an irregular three-dimensional arrangement.
$(4)$ They show a rapid change in volume upon heating.

Which of the following possesses a layered crystal structure?

Which among the following is ferromagnetic in nature?

The sharp melting point of crystalline solids is due to

Classify solids on the basis of their electrical conductivities.