Which of the following is least reactive in a nucleophilic...

Chlorine of vinyl chloride $\left(\mathrm{CH}_2=\right. \mathrm{CHCl}$ ) is non-reactive (less reactive) towards nucleophile in nucleophilic substitution reaction because it shows resonance due to $+\mathrm{M}$ effect of $-\mathrm{Cl}$ atom.
In structure II, Cl-atom have positive charge and partial double bond character with $\mathrm{C}$ of vinyl group, so it is more tightly attracted towards the nucleus and it does not get replaced by nucleophile in $\mathrm{S}_{\mathrm{N}}$ reaction. Related Theory $s p^3$-hybridized electrophile must have a leaving group $(\mathrm{X})$ in order for the reaction to take place.