Consider the following statements
I : In diamond, each carbon atom is linked tetrahedrally for four other carbon atoms by ${sp}^3$ bonds.
II : Graphite has planar hexagonal layers of carbon atoms held together by weak Van der Waal's forces.
III : Silicon exists only in diamond structure due to its tendency to form $p\pi -p\pi$ bonds to itself.

In this -

The correct option is A Only I and II are correct
Structures of graphite, diamond and silicons are shown in figure. In diamond $C$ atom is ${sp}^3$ hybridised due to tetrahedral structure. Graphite has planar hexagonal layers of carbon atoms held together by weak Vander Waals forces and $C$ is ${sp}^2$ hybridised. Electrons are delocalised over the whole sheet. Silicon is covalent, 3-D network solid in which each silicon atom is covalently bonded. It is hard as diamond and non-reactive to to high $Si-O$ bond enthalpy.