Which of the following best explains why the two C–O bond lengths in the formate ion, $
\mathrm{HCO}_{2}^-$, are identical and intermediate between a typical C=O double bond and a C–O single bond?
a) Each C–O bond alternates between single and double character because the bonds rapidly switch positions in time.
b) Because the carbon atom is sp2-hybridized, both C–O bonds must have identical lengths regardless of bond order.
c) Electron delocalization produces a resonance hybrid in which each C–O bond has a fractional bond order between 1 and 2.
d) Steric repulsion between the two oxygen atoms forces the C–O bonds to adopt identical lengths.