When \(\mathrm{RNH}_{2}\) is warmed with \(\mathrm{CS}_{2}\) in presence of \(\mathrm{HgCl}_{2}\), RNCS is formed. This is called (a) Hoffmann exhaustive methylation reaction (b) Hoffmann degradation reaction (c) Hoffmann carbylamine reaction (d) Hoffmann mustard oil reaction

The given reaction can be written as follows: RNH₂ + CS₂ (in presence of HgCl₂) -> RNCS In this reaction, an amine (RNH₂) reacts with carbon disulfide (CS₂) and forms isothiocyanate (RNCS) in the presence of mercury chloride (HgCl₂) as a catalyst.

(a) Hoffmann exhaustive methylation reaction involves the conversion of a primary amine to a tertiary amine by repeated alkylation followed by its rearrangement to form a primary amine with one carbon less in its chain. (b) Hoffmann degradation reaction refers to a reaction that converts an amide to an amine with one carbon less in its chain by converting it to an isocyanate intermediate which then gets hydrolyzed. (c) Hoffmann carbylamine reaction is a test for primary amines in which they react with chloroform in the presence of a strong base like sodium hydroxide to form carbylamines which have a very foul smell. (d) Hoffmann mustard oil reaction involves a reaction between an amine and carbon disulfide in the presence of a catalyst to form isothiocyanate.

By comparing the given reaction with the descriptions of the reactions in the options, we can conclude that the best match is: (d) Hoffmann mustard oil reaction - which involves a reaction between an amine and carbon disulfide in the presence of a catalyst to form isothiocyanate. The reaction given in the exercise is as follows: RNH₂ + CS₂ (in presence of HgCl₂) -> RNCS Thus, this reaction corresponds to the Hoffmann mustard oil reaction.

The correct option is (d) Hoffmann mustard oil reaction.