Solid X is a very hard solid which is an electrical insulator in solid as well as in molten state and has an extremely high melting point. What type of solid is it?
A) Ionic solid
B) Covalent solid
C) Metallic solid
D) Molecular solid

Hint: Solid is one of the states of the matter in which particles are closely packed hence, possess the lowest kinetic energy. The two main types of solids are crystalline solids and amorphous solids. Crystalline solids are further classified as molecular solids, ionic solids, metallic solids, and covalent solids.

Complete step-by-step answer:
In the case of the ionic solids, an ionic bond is present between constituents. Here, option(A) is ionic solid.Here, it is given that X is a hard solid and acts as an insulator in solid as well as molten state, if it is ionic solid then in a molten state it is conductor, therefore X is not ionic solid.

Here, option(B) is covalent solid. In the case of the covalent solids, a covalent bond is present between the atoms. In the case of the covalent solid, there is a strong electrostatic force of attraction therefore it possesses a high melting point. But there is no free electron in the case of the covalent solids in solid as well as molten state therefore, covalent solids act as an insulator in solid as well as the molten state. Hence, option(B) is the correct answer to the question.

Now, option (C) metallic solids. In the case of metallic solids, the metallic bond is present between the metal atoms. Metallic solids contain free-electron, therefore, acts as a conductor, and hence, option(C)is incorrect.

Option(D) molecular solids. In the case of molecular solids, molecules are held together by intermolecular forces. In the case of the molecular solids atoms or molecules are held together by the weaker forces of attraction that is London dispersion forces, dipole-dipole forces hence, molecular solids are poor conductors. Hence, option(D) is also incorrect.

Note: The conductor is the substance that conducts the electricity. The free electrons or ions are responsible for the conduction. If a substance possesses a free electron in solid-state and forms the ions in molten state the ions act as conductors.
Insulators are the substance which does not conduct the electricity because of the absence of free electrons or ions.