Reactivity in nucleophilic substitution reactions is determined by the strength of the bond between the carbon and the leaving group. The weaker the bond, the more reactive the compound will be. In this case, we have four compounds to compare:

a) (CH3)3C-Cl
b) CH2=CHCl
c) CH3CH2Cl
d) CH2=CHCH2Cl

To determine the reactivity, we need to consider the nature of the leaving group and the stability of the carbocation intermediate formed during the reaction.

1. Leaving Group:
The leaving group in all four compounds is a chloride ion (Cl-). Chloride ions are good leaving groups because they are stable and can easily accept a pair of electrons. Therefore, the nature of the leaving group does not determine the reactivity in this case.

2. Carbocation Stability:
The more stable the carbocation intermediate, the more reactive the compound. Carbocation stability depends on the number of alkyl groups attached to the positively charged carbon atom. The order of carbocation stability is as follows:

tertiary > secondary > primary > methyl

a) (CH3)3C-Cl: This compound forms a tertiary carbocation, which is the most stable carbocation. Therefore, it is the most reactive compound.

c) CH3CH2Cl: This compound forms a primary carbocation, which is less stable than a tertiary carbocation. Therefore, it is less reactive than compound (a).

d) CH2=CHCH2Cl: This compound forms a secondary carbocation, which is more stable than a primary carbocation but less stable than a tertiary carbocation. Therefore, it is less reactive than compound (a) but more reactive than compound (c).

b) CH2=CHCl: This compound does not form a carbocation intermediate during the reaction. Instead, it undergoes a concerted mechanism called an SN2 reaction, where the nucleophile attacks the carbon atom and the leaving group leaves simultaneously. This mechanism does not involve the formation of a carbocation intermediate. Therefore, compound (b) is the least reactive among the four compounds.

In summary, compound (b) (CH2=CHCl) is the least reactive in a nucleophilic substitution reaction because it does not undergo carbocation formation and follows an SN2 mechanism.