Pick out  from  following compounds which have a greater tendency to undergo SN¹ reaction

I is vinyl halide ,vinyl halides can't under go SN¹  reaction, because $\mathrm{C}-\mathrm{X}$  bond in vinyl halides gets partial double bond character, hence breaking of $\mathrm{C}-\mathrm{X}$bond is highly difficult;

Moreover the carbocation formed is highly unstable, so vinyl halides can't give SN¹ reaction.

IV is bridged halo compound, bridge head halogen can't be replaced either by  SN¹  or SN².  

The carbocation formed at bridge head position is highly unstable because the attainment of planarity at bridge head position is not possible, it can't undergo SN¹ reaction.

II is 3⁰-alkyl halide and III is benzyl halide having a EDG group at para position. They are highly reactive with respect to SN¹ ,as they form stable carbocations.