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

The correct option is CH₃CH₂OH.

Step-by-Step Explanation:

1. Diazotization (First Step):
The reactant shown on the far left of the image is 2,4,6-tribromoaniline. When treated with sodium nitrite (NaNO₂) and hydrochloric acid (HCl) at a low temperature range of 0 - 5 °C, the amino group (-NH₂) undergoes diazotization to form 2,4,6-tribromobenzenediazonium chloride, as shown in the middle structure of the reaction sequence.

2. Reductive Deamination (Second Step):
In the second step of the reaction, the diazonium group (-N₂⁺Cl^-) is replaced by a hydrogen atom (-H) to yield 1,3,5-tribromobenzene (shown on the far right). This reduction process is known as deamination.

3. Role of Reagent 'R':
To replace the diazonium group with a hydrogen atom, mild reducing agents are used. Common reagents for this transformation are hypophosphorous acid (H₃PO₂) or ethanol (CH₃CH₂OH).
When ethanol (CH₃CH₂OH) is used, it reduces the diazonium salt to the corresponding arene, while itself getting oxidized to ethanal (acetaldehyde, CH₃CHO).

The overall reduction reaction equation is:

$\mathrm{Ar}-N_2^{+}{\mathrm{Cl}}^{-}+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\to \mathrm{Ar}-H+N_2+\mathrm{HCl}+{\mathrm{CH}}_3\mathrm{CHO}$

Here, the symbol Ar represents the 2,4,6-tribromophenyl group.

4. Analysis of other options:
- H₂O: Hot water converts diazonium salts to phenols.
- HI: Replaces the diazonium group with iodine (-I).
- CuCN/KCN: Replaces the diazonium group with a cyano group (-CN) via the Sandmeyer reaction.

Therefore, the reagent 'R' must be ethanol, CH₃CH₂OH.