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

The correct answer is CH₃CH₂OH.

Let us break down the reaction sequence step-by-step to understand the role of each chemical and reagent involved:

Step 1: Diazotisation
The starting material shown in the image is 2,4,6-tribromoaniline, which has three bromine substituents at the 2, 4, and 6-positions and an amino group ($-{\mathrm{NH}}_2$) at the 1-position of the benzene ring. When treated with sodium nitrite (${\mathrm{NaNO}}_2$) and hydrochloric acid ($\mathrm{HCl}$) at $0-5°C$, the amino group undergoes diazotisation to form the diazonium salt:
$\text{2,4,6-tribromobenzenediazonium chloride}$ (as indicated by the $-N_2^{+}{\mathrm{Cl}}^{-}$ group in the intermediate structure).

Step 2: Deamination (Reduction of Diazonium Salt)
In the second step, the diazonium group ($-N_2^{+}{\mathrm{Cl}}^{-}$) is replaced by a hydrogen atom ($-H$) to yield 1,3,5-tribromobenzene. This reduction reaction is known as deamination. Mild reducing agents are required to carry out this conversion. The two most common reagents used for this purpose are:
1. Ethanol (${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$)
2. Hypophosphorous acid ($H_3{\mathrm{PO}}_2$)

When ethanol (${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}$) is used, it reduces the diazonium salt to the corresponding arene while ethanol itself is oxidized to ethanal (acetaldehyde). The general reaction is:
${\mathrm{Ar-N}}_2{\mathrm{Cl}}^{-}+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\to \mathrm{Ar-H}+N_2+\mathrm{HCl}+{\mathrm{CH}}_3\mathrm{CHO}$

Comparing this with the options provided:
• H₂O: Hydrolysis with water replaces the diazonium group with a hydroxyl group ($-\mathrm{OH}$), forming a phenol.
• CH₃CH₂OH: Successfully reduces the diazonium salt to the corresponding hydrocarbon (replacing $-N_2^{+}{\mathrm{Cl}}^{-}$ with $-H$).
• HI: Replaces the diazonium group with an iodine atom ($-I$).
• CuCN/KCN: Replaces the diazonium group with a cyano group ($-\mathrm{CN}$) via the Sandmeyer reaction.

Thus, the reagent R is CH₃CH₂OH.