The brown ring test is a chemical test used to detect the presence of the nitrate ion (NO3-) in a solution. This test is based on the reaction between the nitrate ion and iron(II) ion (Fe2+), which forms a brown-colored complex.

Here's a detailed explanation of the answer:

Introduction to the Brown Ring Test:
The brown ring test is a qualitative test that is widely used in inorganic chemistry to identify the presence of nitrate ions in a solution. This test involves the reaction between the nitrate ion and iron(II) ion in the presence of sulfuric acid. The reaction forms a complex compound known as nitrosyl iron(II) sulfate, which gives a brown ring at the junction of the two liquid layers.

The Reaction:
The reaction involved in the brown ring test can be written as follows:

Fe2+ + 2NO3- + 6H+ → Fe(NO)2+ + 2H2O

In this reaction, iron(II) ions (Fe2+) react with nitrate ions (NO3-) in the presence of an acidic medium (sulfuric acid) to form nitrosyl iron(II) sulfate (Fe(NO)2+). The brown ring is formed due to the complexation of nitrosyl iron(II) sulfate with the nitrate ion.

Why is the Brown Ring Formed?
The brown ring is formed due to the complexation between the nitrate ion and the nitrosyl iron(II) sulfate. The nitrate ion acts as a ligand and coordinates with the central iron atom in the complex. This coordination leads to the formation of a colored complex, which appears as a brown ring at the junction of the two liquid layers.

Which Ion is Detected by the Brown Ring Test?
Out of the given options, the brown ring test is useful for detecting the presence of the nitrate ion (NO3-). This means that the correct answer is option 'D' (NO3-). The brown ring test does not detect nitrite ions (NO2-) or nitrous acid (HNO2).

Conclusion:
The brown ring test is a useful chemical test for detecting the presence of the nitrate ion (NO3-) in a solution. This test relies on the formation of a brown-colored complex between the nitrate ion and iron(II) ion in an acidic medium. By observing the formation of a brown ring at the junction of the two liquid layers, one can conclude the presence of nitrate ions in the solution.