Identify the test being discussed

The question refers to the brown ring test for the nitrate ion, $NO_3^-$.
In this test, the nitrate solution is treated with freshly prepared $FeSO_4$ solution and then concentrated $H_2SO_4$ is carefully added along the sides of the test tube.

What is observed in the nitrate test?

Nitrate ion is reduced to nitric oxide, $NO$, by $Fe^{2+}$ in acidic medium: $NO_3^- + 3Fe^{2+} + 4H^+ \rightarrow NO + 3Fe^{3+} + 2H_2O$

The $NO$ formed then combines with ferrous ion to give a brown ring complex at the junction of the two layers.

Check each option

Option A

A dark brown ring is formed at the junction of two solutions.

This is correct. In the brown ring test, a dark brown ring appears at the interface of the acid layer and aqueous layer.

Option B

Ring is formed due to nitroferrous sulphate complex.

This is not true. The brown ring is due to a nitrosyl complex, not a nitro complex.

• Nitro means ligand $NO_2^-$ attached through nitrogen.
• In the brown ring test, the ligand is nitric oxide, $NO$, so the complex is nitrosyl ferrous sulphate.

Hence this statement is false.

Option C

The brown complex is $[Fe(H_2O)_5(NO)]SO_4$.

This is correct. The brown ring complex is represented as: $[Fe(H_2O)_5(NO)]SO_4$

Option D

Heating the nitrate salt with conc. $H_2SO_4$, light brown fumes are evolved.

This is also true. On heating nitrate salts with concentrated $H_2SO_4$, nitric acid may form and decompose to give brown fumes of nitrogen dioxide, $NO_2$.

Conclusion

The statement which is not true is: $\boxed{\text{B}}$