In molecular crystals, atoms are bounded by weak van der Walls forces. They have no free electrons in them. So, they are electrical insulators.

In ${ H } _{ 2 }\quad and\quad { I } _{ 2 }$, molecules are attracted by weak van der Waals attractions. So, they are molecular solids. 

In the given options, both ionic and metallic solids are electrical conductors and have high melting point. But only metallic solids are malleable and ductile. 

In solid state, atoms in ionic solids are bonded by strong ionic bonds. They are able to move freely. So, they do not conduct electricity in solid state. While in molten form, the ions and electrons are free to move. So, they conduct electricity in molten state.

In the given options, Calcium Fluoride and Sodium Chloride are the ionic crystals due to presence of ionic bond in them.

Covalent solids have high melting point due to their interconnected covalent bonds. Since there are no free electrons present in covalent solids, so they do not conduct electricity in solid state as well as molten state. Some of the examples are Diamond and Quartz.

Since graphite is an allotrope of carbon hence, each atom is linked to each other through a covalent bond. Therefore, graphite is a covalent solid. 

In molten state, the ionic solids have free ions in them. Ions help in the conduction of electricity. Due to these free ions the electrical conductivity of ionic compounds in molten state is very high. 

A network solid or covalent network solid is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual molecules, and the entire crystal or amorphous solid may be considered a macromolecule.

In ionic solids anions (which are normally longer in size than cations forms closed pack array. 

Vander Waal's forces make the molecular crystals, solids at low temperature.

Molecular solid is a solid in which the constituent particles are atoms or molecules which are held together by weak intermolecular forces i.e. van der Waals forces. Due to weak forces, they are volatile.

Potassium chloride KCl is a metal halide salt composed of potassium and chloride. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water and its solutions have a salt-like taste.

Carbon and silicon are chemical elements with atomic number 6 and 14. they are nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. Option-B.

Metallic solids are generally non-brittle or highly malleable and ductile. Also metallic bond is one of the strongest bonds. 

Ionic crystals have very low volatility. Generally, ionic solids are having ionic bonds between the constituent atoms. 

Ionic bonds are very strong bonds. So they are less volatile.

Melting point of an ionic compound is determined by the electrostatic attraction between cations and anions. It takes a lot of energy to overcome this attraction to allow the ions to move freely and form a liquid. The factor witch effect melting point is charge of the ion. Since, $KCl$ has $+1$ and $-1$ charge on its' cations and anions respectively. Hence, it has the lowest melting point.

An ionic solid has some point defect but its experimental density is equal to its theoretical density. The type of defect is Frenkel effect.

When an ion leaves its correct lattice site and occupies an interstitial site, Frenkel defect is observed. Hence, the density of the crystal is not affected.

In a Schottky defect, the density of the crystal decreases.

Hence, the given statement is $\text{true}$

A covalent network crystal consists of atoms at the lattice points of the crystal, with each atom being covalently bonded to its nearest neighbour atoms.

The thermal conductivity of silver and copper is high because they have metallic bonding.

The molecular range for materials is $10^{-9}m$. It can also be expressed in terms of nanometers. $1  nm =10^{-9}m$. 

Both theories have their own arguments to explain metallic bonding.
Each metal atom allows its electrons to roam freely, so these atoms become positively charged cations. These cations are kind of like a positively charged island and are surrounded by a sea of negatively charged electrons.

The following properties are important in determining whether an element has metallic properties.
III: number of valence electrons
IV: number of vacant atomic orbitals
All metals are electropositve elements and have a tendency to lose electrons.  Alkali metals and alkaline earth metals contain one and two valence electrons respectively.They readily lose these electrons and show metallic properties. Transition metals have a number of vacant d orbitals and show variable valency. Due to the presence of vacant d orbitals, they show variable valence.

Molecular solids refer to those solids which are composed of molecules held together by the van der Waals forces. 

Metals are good conductors of heat and electricity with variable melting points.
Ionic solids are also good conductors of heat and electricity with very high melting points.
Covalent solids are non conductor of electricity and heat with very high melting points due to their strong bonding.
Molecular solids are poor conductors of heat and electricity and they have lower melting point than ionic solids.

Molecular crystals exist in all crystalline state, an amorphous state, and non-crystalline state. Molecular crystals are substances that have relatively weak intermolecular binding, such as dry ice (solidified carbon dioxide), solid forms of the rare gases (e.g., argon, krypton, and xenon).

Generally, ionic compounds exist in solid state due to strong electrostatic ionic forces present between the molecules of solids.

Metals are opaque because they have metallic bonding which means that all of the atoms are surrounded by free-moving electrons.

Covalent solids form crystals that can be viewed as a single giant molecule made up of an almost endless number of covalent bonds. 

In graphite, each carbon atom uses only 3 of its 4 outer energy level electron in covalently bonding to there other carbon atoms in a place. In graphite, electrons are spread out between the structure.

Pressure and temperature also influence the structure of ionic solids. The structures of chlorides, bromides and iodides of sodium, potassium and rubidium having $6:6$ Co-ordination at normal temperature and pressure changes to $8:8$ co-ordination like CsCl structure at high pressure, Thus, high pressure increases co-ordination number. Furthermore, CsCl structure changes to NaCl structure at $760$ K. Thus, high temperature decreases co-ordination number.

Molecular Crystals: 

Melting point increases with increase in pressure. This is because , due to increased pressure molecules tend to remain in solid state. (This is the basic principle on which liquification of gas works)

In ionic solids, atoms are present as ions, where cations and anions are arranged in various structure giving rise to a variety of lattices. Thus, they do not have molecules.

Molecular solid is a solid composed of molecules held together by the van der Waals forces. Because these dipole forces are weaker than covalent or ionic bonds, molecular solids are soft and have relatively low melting temperature. Pure molecular solids are electrical insulators but they can be made conductive by doping. Examples of molecular solids include hydrocarbons, ice, sugar, fullerenes, sulfur and solid carbon dioxide.

More is the deformation in anion, more is the covalent character. 
Pseudo inert gas cation e.g. $Cd^{2+}$ causes more polarisation of ion and so, it has higher covalent character.

Covalent crystals are formed by sharing of valence electrons between two atoms resulting in the formation of a covalent bond. 

A network solid or covalent network solid is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual molecules, and the entire crystal may be considered a macromolecule. $SiO _2$ is a network solid. $Li _2O$ and $NaCl$ are ionic crystals. $H _2O$ is a covalent liquid and $CO _2$ is a covalent gas.

Carbon dioxide $\displaystyle CO _2 $ has polar bonds but is a nonpolar molecule. The structure of $\displaystyle  CO _2$  is linear. The individual bond dipoles cancel each other as they point in opposite direction and are equal in magnitude. Hence, the molecule is non polar with zero dipole moment.

$Li (s)$ is a metallic solid and has metallic network involving strong metallic bonds. It is crystalline solid formed by atoms of $Li$ metal.

Covalent-network (also called atomic) solids—Made up of atoms connected by covalent bonds; the intermolecular forces are covalent bonds as well. Characterized as being very hard with very high melting points and being poor conductors. Examples of this type of solid are diamond and graphite, and the fullerenes. 

In the ionic solid ammonium nitrate $NH _4NO _3$,  ions present are ammonium $NH _4^+$   and nitrate $NO^- _3$. Strong electrostatic force of attraction is present between oppositely charged ions.

Ionic solids are composed of oppositely charged ions(i.e. positively charged cations and negatively charged anions). There is a strong force of attraction because they are oppositely charged. Hence a lot of energy is required to break them due to which they have high melting point.

Metallic bonding is responsible for the stability of some solid material to conduct electricity. These bonding have delocalized electrons which act as charge carriers and hence conduct electricity.

In the solid-state ionic compounds are held together by strong electrostatic forces. So, the conduction of electricity is not possible. But in the molten state, the ions are free to move and hence conduct electricity.

At $20^\circ C$,

A Covalent network solid is a chemical compound in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual molecules, and the entire crystal or amorphous solid may be considered a macromolecule.

Diamond is a metastable allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Graphite is a crystalline form of carbon and one of the allotropes of carbon. Graphite is the most stable form of carbon under standard conditions. It has a layered, planar structure. The individual layers are called graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice with separation of $0.142 nm,$ and the distance between planes is $0.335 nm.$ Correct answer is option-B.

The major binding force of diamond, silicon and quartz is covalent bond force.
For example, diamond is a covalent solid. It is a big giant network of $\displaystyle sp^3$ hybridised C atoms linked through strong covalent bonds.

Geomatrical shape of crystal can be calculated by knowing the radius ratio in a crystal of ionic solid.

Graphite is a mineral made of loosely bonded sheets of carbon atoms, giving it a slippery texture that makes it a very effective lubricant.

In $2:3$ spinel structure                                        General formula is $A^{+2}B _2^{+3}O _4$

Graphite is covalent solid. Strong covalent bonds are present in between  C atoms in graphite. Another example of a covalent solid is diamond 

Metallic bonding: The chemical bonding that takes place from the attraction of metal atoms and the surrounding sea of electrons.
Metallic bonding is the force of attraction between valence electrons and metal ions. It is the sharing of many detached electrons between many positive ions, where the electrons act as a "glue" giving the substance a definite structure. It is unlike covalent or ionic bonding

The electrons and the positive ions in the metal have a strong attractive force between them. Therefore, metals often have high melting or boiling points. The principle is similar to that of ionic bonds.

Crystal structures of various systems are:

$K _2Cr _2O _7$ - Triclinic

$NaAlF _6$ - Monoclinic

$NaCl$ - Cubic unitcell (Face centered)

Quartz – Triagonal