Explanation:

Nucleophilic substitution reactions can proceed via two mechanisms: SN1 (substitution nucleophilic unimolecular) and SN2 (substitution nucleophilic bimolecular). The choice of mechanism depends on the nature of the substrate and the nucleophile.

SN1 Mechanism:
- In SN1 mechanism, the reaction proceeds in two steps: the formation of a carbocation intermediate followed by the attack of a nucleophile on the carbocation.
- This mechanism is favored when the substrate is a tertiary alkyl halide or a benzylic/allylic halide.
- The rate of the reaction depends only on the concentration of the substrate, as the nucleophile attacks the carbocation formed independently of its concentration.
- The leaving group leaves before the nucleophile attacks, resulting in a racemic mixture of products if the carbocation is chiral.

SN2 Mechanism:
- In SN2 mechanism, the reaction proceeds in a single step where the nucleophile attacks the substrate and displaces the leaving group.
- This mechanism is favored when the substrate is a primary or methyl alkyl halide.
- The rate of the reaction depends on both the concentration of the substrate and the nucleophile, as the nucleophile directly attacks the substrate.
- The stereochemistry of the product is inverted (Walden inversion) due to the backside attack of the nucleophile.

Analysis of the options:
a) Ethyl chloride - Ethyl chloride is a primary alkyl halide, and primary halides typically undergo nucleophilic substitution via the SN2 mechanism. Therefore, it does not exclusively undergo SN1 mechanism.
b) Isopropyl chloride - Isopropyl chloride is a secondary alkyl halide, and secondary halides can undergo nucleophilic substitution via both SN1 and SN2 mechanisms. Therefore, it does not exclusively undergo SN1 mechanism.
c) Chlorobenzene - Chlorobenzene is an aryl halide, and aryl halides do not undergo nucleophilic substitution via either SN1 or SN2 mechanisms. Therefore, it does not undergo SN1 mechanism.
d) Benzyl chloride - Benzyl chloride is a benzylic halide, which is a type of substrate that prefers the SN1 mechanism. The benzyl carbocation formed in the first step is stabilized by resonance, making the SN1 mechanism more favorable. Therefore, it exclusively undergoes nucleophilic substitution via the SN1 mechanism.

Therefore, the correct answer is option 'D' - Benzyl chloride.