Nucleophilic substitution reactions involve the replacement of a nucleophile (an electron-rich species) with a leaving group (an atom or group of atoms that departs with a pair of electrons) in a molecule. The halogen compounds typically undergo nucleophilic substitution reactions.

The given options are:

a) CH2=CHCl
b) CH3CH=CHCl
c) CH2=CHC(Cl)=CH2
d) CH2=CHCH2Cl

To determine which compound undergoes nucleophilic substitution most readily, we need to consider the reactivity of the leaving group and the stability of the carbocation intermediate formed during the reaction.

1. Reactivity of the Leaving Group:
The leaving group is the halogen atom in all the given compounds. Halogens are good leaving groups because they are electronegative and can stabilize the negative charge on the halide ion formed after leaving. However, the reactivity of the leaving group decreases in the order: I > Br > Cl > F. This means that compounds with iodine as the leaving group will undergo nucleophilic substitution more readily than compounds with bromine, chlorine, or fluorine as the leaving group.

2. Stability of the Carbocation Intermediate:
During nucleophilic substitution, a carbocation intermediate is formed when the leaving group departs. The stability of the carbocation determines the ease of nucleophilic substitution. The more stable the carbocation, the more readily the compound undergoes nucleophilic substitution.

Now let's analyze each option:

a) CH2=CHCl:
This compound has a chlorine atom as a leaving group. Chlorine is less reactive than iodine, bromine, and fluorine as a leaving group. Also, the formation of a carbocation intermediate in this compound would involve destabilizing the double bond. Therefore, this compound is less likely to undergo nucleophilic substitution.

b) CH3CH=CHCl:
Similar to option a, this compound has a chlorine atom as a leaving group. The presence of the double bond also makes the carbocation intermediate less stable. Hence, this compound is less likely to undergo nucleophilic substitution.

c) CH2=CHC(Cl)=CH2:
This compound has a chlorine atom as a leaving group. However, the presence of the double bond conjugated with the leaving group provides extra stability to the carbocation intermediate. Therefore, this compound is more likely to undergo nucleophilic substitution compared to options a and b.

d) CH2=CHCH2Cl:
This compound has a chlorine atom as a leaving group. The presence of the alkyl group (CH2) adjacent to the leaving group increases the stability of the carbocation intermediate. Therefore, this compound is the most likely to undergo nucleophilic substitution among the given options.

In conclusion, option d (CH2=CHCH2Cl) undergoes nucleophilic substitution most readily due to the stability of the carbocation intermediate.