What is the state of hybridisation of carbon in (a) $C{O}_3^{2-}$ (b) diamond (c) graphite?

The state of hybridisation of carbon in:

(a) $C{O}_3^{2-}$

C in $C{O}_3^{2-}$ is $s{p}^2$ hybridised and is bonded to three oxygen atoms.

(b) Diamond

Each carbon in diamond is $s{p}^3$ hybridised and is bound to four other carbon atoms.

(c) Graphite Each carbon atom in graphite is $s{p}^2$ hybridised and is bound to three other carbon atoms.