Dehydrohalogenation of an alkyl halide is a/an

$A$ bromoalkane $X$ reacts with magnesium in dry ether to form compound $Y$. The reaction of $Y$ with methanal followed by hydrolysis yields an alcohol having molecular formula $C_{4}H_{10}O$. The compound $X$ is

Which of the following is most reactive towards alcoholic $KOH$?

Observe the following reactions:
$1$. $C_6H_5-CH=CH_2 + HBr \rightarrow X$
$2$. $C_6H_5-C(CH_3)=CH_2 + HBr \rightarrow Y$
$3$. $C_6H_5-CH=CH_2 + HBr \xrightarrow{(C_6H_5COO)_2} Z$
The correct order of reactivity of $X, Y, Z$ towards $S_N1$ reaction is:

Which among the following compounds undergoes $SN^2$ reaction most rapidly?

For $(i) \, I^{-}, \, (ii) \, Cl^{-}, \, (iii) \, Br^{-}$,the increasing order of nucleophilicity would be