Assertion: Graphite is an example of a tetragonal crystal system.
Reason: For a tetragonal system, $ \left( a=b\ne c \right);\left( \alpha =\beta =90{}^\circ \right);\left( \gamma =120{}^\circ \right). $
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion.
(C) Assertion is correct but reason is incorrect.
(D) Both assertion and reason are incorrect.

Hint :We know that we have to have the knowledge about crystal systems and their structures. Knowing the structures we can know whether the angles and lengths are equal or not. Crystallography is the study of crystals systems and lattice systems.

Complete Step By Step Answer:
Here, it is important that we discuss the tetragonal crystal system in detail. Let’s first start with talking about crystallography and crystal systems. In crystallography, the Crystal system, crystal family and lattice system refers to several classes of space groups, lattices, point groups and crystals. They are commonly confused with each other but are slightly different from each other. There are seven crystal systems which are Isometric.
Tetragonal, Orthorhombic, Hexagonal, Triclinic, Monoclinic and Rhombohedral crystal systems. Tetragonal crystal system is one of the seven crystal systems. Tetragonal Crystal lattices are formed as a result of stretching a cubic lattice along one of its lattice vectors. So, a cube becomes a rectangular prism with a square base. Examples of tetragonal crystal properties include apophyllite, zircon, rutile quartz whereas graphite is an example of hexagonal system.
Graphite is an example of hexagonal system for which $ \alpha =\beta ={{90}^{\circ ~}} $ and $ \gamma =120{}^\circ $ and $ a=b\ne c.~ $
So, Assertion is false. Reason is also false as for the tetragonal system $ a=b\ne c~ $ and $ \alpha =\beta =\gamma ={{90}^{\circ }}. $
Therefore option C is correct, both assertion and reason are incorrect.

Note :
Remember that a unit cell is the smallest three-dimensional portion which repeats itself in different directions to produce a complete space lattice with characteristics of a unit cell. A unit cell is characterized by: Its dimension along the three edges a, b and c. These edges may or may not be mutually perpendicular. Angles between the edges $ \left( \alpha ,\beta ,\gamma \right). $