What is the state of hybridisation of carbon in $(a)$ $CO_3^{2-}$,$(b)$ diamond,and $(c)$ graphite?

(N/A) $CO_3^{2-}$: The carbon atom in $CO_3^{2-}$ is $sp^2$ hybridised,forming three sigma bonds with oxygen atoms.
$(b)$ Diamond: Each carbon atom in diamond is $sp^3$ hybridised,forming four sigma bonds with four other carbon atoms in a tetrahedral geometry.
$(c)$ Graphite: Each carbon atom in graphite is $sp^2$ hybridised,forming three sigma bonds with three other carbon atoms in a planar hexagonal layer structure.