Which of the following alkenes is the most reactive towards cationic polymerization?
 ${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_3, {\mathrm{CH}}_2=\mathrm{CH}-\mathrm{Cl}, {\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{C}}_6{\mathrm{H}}_5, {\mathrm{CH}}_2={\mathrm{CHCOOCH}}_3$

The reactivity of alkenes towards cationic polymerization increases as the stability of the intermediate carbocation increases.

In ${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{C}}_6{\mathrm{H}}_5,$​ the carbocation is stabilized by resonance, thus is most reactive towards cationic polymerization.