Which of the following is a reagent used in the preparation of benzene diazonium chloride from aniline?

Sodium nitrite

The reagent used in the preparation of benzene diazonium chloride from aniline is sodium nitrite \((NaNO_2)\). So the answer is (3).

The other reagents listed are not used in the preparation of benzene diazonium chloride. Sodium nitrate is used to make nitric acid, which is a reactant in the diazotization reaction. Sodium hydroxide is used to make a basic solution, which is necessary for the diazotization reaction. Sodium chloride is not used in the diazotization reaction.

Here is the equation for the diazotization reaction:

In this reaction, aniline reacts with sodium nitrite and hydrochloric acid to form benzene diazonium chloride and sodium chloride. The water is a byproduct of the reaction.