Identify weak field ligand from following.

Ligand field strength is determined by the spectrochemical series: $\mathrm{I}^{-} < \mathrm{Br}^{-} < \mathrm{S}^{2-} < \mathrm{SCN}^{-} < \mathrm{Cl}^{-} < \mathrm{NO}_{3}^{-} < \mathrm{F}^{-} < \mathrm{OH}^{-} < \mathrm{C}_{2}\mathrm{O}_{4}^{2-} < \mathrm{H}_{2}\mathrm{O} < \mathrm{NCS}^{-} < \mathrm{EDTA}^{4-} < \mathrm{NH}_{3} < \mathrm{en} < \mathrm{NO}_{2}^{-} < \mathrm{CN}^{-} < \mathrm{CO}$

Among the options, $\mathrm{NH}_{3}$ is a strong field ligand, $\mathrm{NC}^{-}$ is a very strong field ligand (similar to $\mathrm{CN}^{-}$), $\mathrm{Br}^{-}$ is a weak field ligand, and $\mathrm{CO}$ is the strongest ligand in the series. The weakest is $\mathrm{Br}^{-}$

Answer: $\boxed{\text{C}}$