Identify the halogen exchange reaction from the following.

Which of the following is least reactive towards $\mathrm{S}_{\mathrm{N}} 1$ reactions?

An alkyl halide $X\left(\mathrm{C}_4 \mathrm{H}_9 \mathrm{Br}\right)$ undergoes nucleophilic substitution by $\mathrm{S}_{\mathrm{N}} 2$ reaction. The product of $X$ on reaction with $\mathrm{Mg} /$ dry ether followed by $\mathrm{D}_2 \mathrm{O}$ is

Choose the correct decreasing order of reactivity of alkyl halides towards $\mathrm{S}_{\mathrm{N}} 1$ reaction.