Which of the following undergoes nucleophilic substitution by $S_N1$ mechanism?

Observe the following reactions. The order of reactivity of $x, y, z$ towards $S_{N}1$ reaction is:

Describe the chemical reactions of haloalkanes.

The compound most reactive towards $S_{N}1$ reaction is :

An alkyl halide $X$ $(C_4H_9Br)$ undergoes nucleophilic substitution by $S_N2$ reaction. The product of $X$ on reaction with $Mg/\text{dry ether}$ followed by $D_2O$ is:

What is the product of the following reaction?
$C_6H_5CH_2Br + KF \xrightarrow{DMF} ?$