The brown ring observed after the addition of a stream of concentrated tetraoxosulphate (VI) acid confirmed the presence of?

To a solution of an unknown compound, a little dilute tetraoxosulphate (VI) acid was added with some freshly prepared iron (II) tetraoxosulphate (VI) solution. The brown ring observed after the addition of a stream of concentrated tetraoxosulphate (VI) acid confirmed the presence of

A. CO-
B. Cl-
C. SO-
D. NO

Correct Answer:

Option D – NO