(a) C in $CO_3^{–2}$ ion is $sp^2$ hybridised. This can be explained as : (b) Diamond : C is $sp^3$ hybridised in diamond. This gives it a tetrahedral structure rendering it hard. (c) Graphite : C is $sp^2$ hybridised in graphite. It forms 3 bonds with 3 other carbon atoms in the same plane while the fourth covalency is satisfied by p-bonding. These p-electrons are delocalised over the whole sheet.