Diamond is harder than graphite because:
(A) graphite is planar
(B) diamond has free electron
(C) graphite has $s{{p}^{3}}$ hybridization
(D) none of these

Hint: Diamond and graphite are allotropes of carbon which have a different way of bonding to the adjacent carbon atom. The structure of the allotropes can be understood by the way the carbon atoms bond to each other. In graphite, carbon attaches to only 3 other carbon atoms. However, in diamond, the carbon atom attaches to 4 carbon atoms hence fulfilling its valency.

Complete step-by-step answer:
Diamond is a solid form of carbon element with its atoms arranged in a crystal lattice called diamond cubic. At room temperature and pressure, carbon exists in another solid form called graphite which is the stable form chemically.
Although graphite is the stable form, diamond almost never converts to graphite form. Diamond has the highest hardest and characteristic thermal conductivity of any material. This property is used for cutting and polishing tools.
Graphite which is also an allotrope of carbon is a good conductor of electricity and heat. Carbon atoms in graphite are attached to only 3 carbon atoms unlike 4 in diamond. Due to this carbon atoms have a free electron. These free electrons are present in sheets of graphite and help in conduction of electricity.
In terms of tensile strength, diamond is much stronger than graphite. This is because diamond exists in crystalline form and graphite is an amorphous solid.

Since none of the given options match the reason for diamond's strength, the correct answer is option (D).

Note: Diamond is a covalent solid and yet has a high melting point mainly due to its interlinked structure. Diamond crystallises in a lattice structure, indicating the presence of strong covalent bonds.
Diamond has a characteristic 3D network in which one carbon is bonded to 4 carbon atoms i.e. having $\text{s}{{\text{p}}^{\text{3}}}$ hybridisation. This network between carbon atoms is the reason why diamond has a high melting point unlike other covalent solids.