Correct option (d)
Benzyl chloride Explanation : SN1 reaction is favoured by heavy (bulky) groups on the carbon atom attached to halogens and nature of carbonium ion in substrate is
Benzyl > Allyl > Tertiary > Secondary > Primary > Methyl halides.

Introduction:
Nucleophilic substitution reactions occur when a nucleophile replaces a leaving group in a molecule. The rate of these reactions depends on several factors, such as the stability of the carbocation intermediate formed during the reaction. In this case, we need to determine which of the given compounds will undergo nucleophilic substitution by Sn1 at the fastest rate.

Explanation:
To determine the rate of nucleophilic substitution reactions, we need to consider the stability of the carbocation intermediate formed. The more stable the carbocation, the faster the reaction will occur. Let's analyze each compound individually:

1. Ethyl chloride:
- Ethyl chloride is a primary alkyl halide, meaning that the carbocation formed will be primary.
- Primary carbocations are highly unstable due to the lack of alkyl groups attached to the positively charged carbon.
- Hence, the reaction rate of nucleophilic substitution by Sn1 will be slow for ethyl chloride.

2. Isopropyl chloride:
- Isopropyl chloride is a secondary alkyl halide, meaning that the carbocation formed will be secondary.
- Secondary carbocations are more stable than primary carbocations because they have two alkyl groups attached to the positively charged carbon.
- The reaction rate of nucleophilic substitution by Sn1 will be faster for isopropyl chloride compared to ethyl chloride.

3. Benzyl chloride:
- Benzyl chloride is a tertiary alkyl halide, meaning that the carbocation formed will be tertiary.
- Tertiary carbocations are the most stable among primary, secondary, and tertiary carbocations because they have three alkyl groups attached to the positively charged carbon.
- The reaction rate of nucleophilic substitution by Sn1 will be faster for benzyl chloride compared to isopropyl chloride.

4. Chlorobenzene:
- Chlorobenzene does not have an alkyl group attached to the carbon bearing the leaving group.
- A carbocation cannot be formed in this case because the aromatic ring provides significant stability to the molecule.
- Therefore, chlorobenzene does not undergo nucleophilic substitution by Sn1.

Summary:
Among the given compounds, benzyl chloride will undergo nucleophilic substitution by Sn1 at the fastest rate. This is because benzyl chloride forms a tertiary carbocation, which is the most stable among the given options. Isopropyl chloride, a secondary alkyl halide, will have a slower reaction rate. Ethyl chloride, a primary alkyl halide, will have an even slower rate. Chlorobenzene, on the other hand, does not undergo Sn1 substitution due to the stability of the aromatic ring.