The reagent ‘R’ in the given sequence of chemical reaction is :

The correct option is CH₃CH₂OH.

Analysis of the Chemical Reaction:

In the given reaction scheme:

Step 1: Diazotization

The starting material is 2,4,6-tribromoaniline. When treated with sodium nitrite and hydrochloric acid (${\text{NaNO}}_2+\text{HCl}$) at low temperature ($0-5{°}^{\text{C}}$), the primary aromatic amine ($-{\text{NH}}_2$) group undergoes diazotization to form 2,4,6-tribromobenzenediazonium chloride containing the diazonium group ($-{\text{N}}_2^{+}{\text{Cl}}^{-}$).

Step 2: Deamination / Reduction

In the second step, the diazonium group ($-{\text{N}}_2^{+}{\text{Cl}}^{-}$) is completely replaced by hydrogen ($-\text{H}$) to give 1,3,5-tribromobenzene.

To reduce a diazonium salt to benzene (or hydride substitution), mild reducing agents such as ethanol (${\text{CH}}_3{\text{CH}}_2\text{OH}$) or hypophosphorous acid (${\text{H}}_3{\text{PO}}_2$) are used. During this process, ethanol acts as a reducing agent and itself gets oxidized to ethanal (acetaldehyde):

$\text{Ar}-{\text{N}}_2^{+}{\text{Cl}}^{-}+{\text{CH}}_3{\text{CH}}_2\text{OH}\to \text{Ar}-\text{H}+{\text{N}}_2+\text{HCl}+{\text{CH}}_3\text{CHO}$

Therefore, the reagent 'R' required for this transformation is CH₃CH₂OH.