= Solution
The relevant <spectrochemical series> order is O-bound nitrito $<$ <ammonia> $<$ N-bound nitro. A <pi-donor ligand> tends to raise the metal orbitals derived from $t_{2g}$, reducing the <octahedral splitting>; a <pi-acceptor ligand> stabilizes those orbitals, increasing the splitting. Since <photon energy> is $E=hc/\lambda$, the stronger-field complex absorbs at the shorter <wavelength>. Thus
$$
\boxed{\lambda_C=458\ \mathrm{nm},\qquad
\lambda_{[\mathrm{Co(NH_3)_6}]^{3+}}=474\ \mathrm{nm},\qquad
\lambda_B=486\ \mathrm{nm}.}
$$
The corresponding <spectroscopic wavenumbers> are approximately $21834$, $21097$ and $20576\ \mathrm{cm^{-1}}$. These are the observed band energies, not three exact independent determinations of $\Delta_o$: low-spin $d^6$ transition energies also contain <electron repulsion> contributions, and replacing one ligand lowers the ideal octahedral <symmetry>. The relative assignment is nevertheless robust.
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