(a) C in CO3–2 ion is sp2 hybridised. This can be explained as :

(b) Diamond : C is sp3 hybridised in diamond. This gives it a tetrahedral structure rendering it hard.
(c) Graphite : C is sp2 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.