Explanation

(a)
In diamond each carbon atom is linked to four other carbon atoms by sigma bond. Each $\sigma$ C-C bond is formed by the overlapping of $sp^{3}$ hybrid orbitals of each carbon atom. Each carbon atom is present at the centre of a regular tetrahedron.
Each carbon atom is surrounded by four other carbon atoms present at the corners of a regular tetrahedron. Structure of diamond is a rigid three dimensional network. This explain high density and hardness of diamond.