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

The correct option is CH₃CH₂OH.

Let us analyze the chemical reaction sequence step-by-step:

Step 1: Diazotization reaction
The starting reactant shown in the image is 2,4,6-tribromoaniline, which features a primary aromatic amino group (${\text{NH}}_2$) on the benzene ring and three bromine ($\text{Br}$) substituents at the 2, 4, and 6 positions.
When treated with sodium nitrite (${\text{NaNO}}_2$) and hydrochloric acid ($\text{HCl}$) at a low temperature of 0 - 5 °C (as written over the first reaction arrow), the amino group undergoes diazotization. This reaction converts the aniline derivative into the corresponding diazonium salt, 2,4,6-tribromobenzenediazonium chloride, which contains the active ${\text{N}}_2^{+}{\text{Cl}}^{-}$ group as labeled in the second chemical structure.

Step 2: Reduction of Diazonium Salt (Deamination)
In the second step of the sequence, the diazonium group (${\text{N}}_2^{+}{\text{Cl}}^{-}$) is replaced by a hydrogen atom ($\text{H}$) to form 1,3,5-tribromobenzene (the final product shown). This reaction is a reductive deamination.
To reduce the diazonium salt, we need a mild reducing agent that can donate a hydride ion. A classic reagent for this transformation is ethanol (${\text{CH}}_3{\text{CH}}_2\text{OH}$).
During this reaction, ethanol acts as the reducing agent and undergoes oxidation to form acetaldehyde (${\text{CH}}_3\text{CHO}$), while the diazonium ion is reduced to nitrogen gas (${\text{N}}_2$) along with the formation of hydrochloric acid ($\text{HCl}$).

The general equation for this step is:
${\text{ArN}}_2\text{Cl}+{\text{CH}}_3{\text{CH}}_2\text{OH}\to \text{ArH}+{\text{N}}_2+{\text{CH}}_3\text{CHO}+\text{HCl}$
(where $\text{Ar}$ represents the 2,4,6-tribromophenyl group).

Therefore, the reagent ‘R’ required to carry out this conversion is ethanol, represented by CH₃CH₂OH.