Which of the following is least reactive in a nucleophilic substitution reaction :-

In vinyl chloride, $CH_2=CH-Cl$, the lone pair on chlorine is in conjugation with the C=C double bond (resonance): $CH_2=CH-\ddot{C}l \leftrightarrow \overset{-}{C}H_2-CH=Cl^+$.
This gives the C–Cl bond partial double bond character, making it shorter and stronger and harder to break.
The carbon is also $sp^2$ hybridised (more electronegative than $sp^3$), and the vinyl cation that would form is very unstable. An $S_N2$ backside attack is also hindered by the $\pi$ electron cloud.
Ethyl chloride reacts by $S_N2$, allyl chloride is very reactive because the allyl cation is resonance stabilised, and tert-butyl chloride reacts readily by $S_N1$ through a stable $3^\circ$ carbocation.
So vinyl chloride, $CH_2=CHCl$, is the least reactive in nucleophilic substitution.