In the brown ring test for nitrate, the brown ring at the junction of the two liquid layers is due to

1. A$[\mathrm{Fe}(\mathrm{H}{}_2\mathrm{O}){}_5\mathrm{NO}]{}^{2+}$
2. B$\mathrm{Fe}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]{}_3$
3. C$\mathrm{Fe}(\mathrm{OH}){}_3$
4. D$\mathrm{Na}{}_4[\mathrm{Fe}(\mathrm{CN}){}_5\mathrm{NOS}]$