A `Aunderset((ii) CN^(-))overset((i)Cl_(2)Delta)to`
(iii) `H_(2)O"/"H^(+)` 4-Bromophenyl acetic acid.
In the above reaction 'A' is

Options

A.
B.
C.
D.

Explanation

To determine the reactant 'A' that leads to the formation of 4-Bromophenylacetic acid through the given reactions, we can break down the steps as follows:

### Step-by-Step Solution:

1. **Identify the Final Product**:
The final product is 4-Bromophenylacetic acid. This compound has a bromine atom at the para position (4-position) of the phenyl ring and an acetic acid functional group.

2. **Analyze the Reagents**:
The reagents provided are:
- Cl₂ (chlorine gas) with heat
- CN⁻ (cyanide ion)
- H₂O/H⁺ (acidic hydrolysis)

3. **Determine the Starting Material**:
To obtain 4-Bromophenylacetic acid, we need to consider the first step involving chlorination. The presence of bromine in the final product suggests that we need a compound that can undergo chlorination to introduce a chlorine atom at the para position.

4. **Propose the Initial Compound**:
A suitable starting compound that contains a bromine atom and can undergo chlorination is 4-Bromotoluene (p-bromotoluene). The structure of 4-Bromotoluene is:
- A benzene ring with a bromine atom at the para position and a methyl group (–CH₃) at the ortho position.

5. **First Reaction - Chlorination**:
When 4-Bromotoluene is treated with Cl₂ in the presence of heat, the methyl group can be chlorinated to form 4-Bromo-1-chlorotoluene. The chlorine atom will replace one of the hydrogen atoms on the methyl group.

6. **Second Reaction - Nucleophilic Substitution**:
The next step involves the reaction with CN⁻. The cyanide ion will perform a nucleophilic substitution reaction, replacing the bromine atom at the para position to form 4-Cyanotoluene.

7. **Final Reaction - Hydrolysis**:
Finally, when 4-Cyanotoluene is treated with H₂O in the presence of acid (H⁺), hydrolysis occurs, converting the nitrile group (–C≡N) into a carboxylic acid group (–COOH), resulting in the formation of 4-Bromophenylacetic acid.

### Conclusion:
Thus, the reactant 'A' is **4-Bromotoluene**.