More the stability of the carbocation, higher will be the reactivity of the parent chloride.

The most reactive compound towards nucleophilic substitution reaction is option D, ClCH2-CH=CH2. Here is the detailed explanation:

Nucleophilic substitution reactions involve the substitution of one nucleophile for another group in a molecule. The reactivity of a compound towards nucleophilic substitution depends on several factors, including the nature of the leaving group, the stability of the carbocation intermediate, and the presence of electron-withdrawing or electron-donating groups.

In this case, let's analyze each option and see why option D is the most reactive:

a) CH2=CH-Cl: This compound contains a vinyl chloride group. Vinyl halides are generally less reactive towards nucleophilic substitution reactions because the carbon-carbon double bond stabilizes the molecule and reduces the electrophilic character of the carbon attached to the halogen. Therefore, option A is less reactive.

b) C6H5Cl: This compound is a simple aryl chloride. Aryl halides are generally unreactive towards nucleophilic substitution reactions due to the resonance stabilization of the aryl group. The delocalization of electrons in the benzene ring makes it difficult for a nucleophile to attack the carbon attached to the chlorine. Therefore, option B is less reactive.

c) CH3CH=CH-Cl: This compound contains a vinylic chloride group similar to option A. The presence of the double bond reduces the reactivity towards nucleophilic substitution reactions. Although it is slightly more reactive than option A due to the presence of the alkyl group, it is still less reactive compared to option D.

d) ClCH2-CH=CH2: This compound contains a primary alkyl chloride group. Primary alkyl halides are generally more reactive towards nucleophilic substitution reactions because the carbon attached to the halogen is less hindered and the carbocation intermediate is relatively stable. In this case, the presence of two alkyl groups adjacent to the chlorine further enhances the reactivity, making option D the most reactive among the given options.

In summary, option D, ClCH2-CH=CH2, is the most reactive towards nucleophilic substitution reactions due to the presence of a primary alkyl chloride group with adjacent alkyl groups, which stabilize the carbocation intermediate and enhance the reactivity.