Which of the following halides is least able to undergo an SN1 reaction?

SN1 needs a carbocation to form on ionisation. Ionisation of vinyl chloride would give a vinyl cation (sp hybridised carbon, positive charge on a vinylic carbon), which is very unstable; in addition the C-Cl bond is shortened by partial double-bond character. The other three give tertiary or triaryl-stabilised cations.