Which of the following alkyl halide will undergo $S_N1$ reaction fastest?

$Me_3C-I$

The correct answer is Option (4) → $Me_3C-I$

For an SN1 reaction, the rate depends mainly on the stability of the carbocation formed and how easily the leaving group departs.

All the given compounds form the same carbocation (tert-butyl carbocation), so the deciding factor is the leaving group ability of the halide:

Leaving group ability order:
F⁻ < Cl⁻ < Br⁻ < I⁻

Thus, tert-butyl iodide (Me₃C–I) undergoes SN1 reaction fastest.