Heading: Introduction

Nucleophilic substitution reaction is a type of organic reaction in which a nucleophile replaces a leaving group in a molecule.

Heading: Factors affecting the reactivity of nucleophilic substitution reactions

The reactivity of nucleophilic substitution reactions is affected by the following factors:

1. Nature of the substrate: The nature of the substrate determines the ease of the formation of the carbocation intermediate. The more stable the carbocation intermediate, the more reactive the substrate will be towards nucleophilic substitution reactions.

2. Leaving group ability: The leaving group ability of the group being replaced determines the rate of the reaction. A good leaving group leaves easily, making the reaction faster.

3. Nature of the nucleophile: The nature of the nucleophile determines its ability to attack the carbocation intermediate. A more reactive nucleophile will react faster.

Heading: Explanation of the correct answer

Out of the given options, ClCH2-CH=CH2 is the most reactive towards nucleophilic substitution reaction. This is because:

1. The substrate contains a primary carbon atom, which means that the carbocation intermediate formed will be relatively unstable. This makes the substrate more reactive towards nucleophilic substitution reactions.

2. The leaving group, Cl-, is a good leaving group, which means that it leaves easily, making the reaction faster.

3. The nucleophile will attack the carbocation intermediate at the primary carbon atom, which is the most accessible site for the nucleophile to attack.

Hence, option D (ClCH2-CH=CH2) is the most reactive towards nucleophilic substitution reaction among the given options.