Which of the following alkyl halides will undergo $S_{N}1$ reaction most readily?

All the given compounds are tertiary alkyl halides but the bond formed between carbon and iodine (C - I) is the weakest bond due to large difference in the size of carbon and iodine. So, ${\left({\mathrm{CH}}_3\right)}_3\mathrm{C}-\mathrm{I}$ gives $S_{N}1$ reaction most readily. In other words, iodine is better
leaving group.