The rate of $\beta$-dehydrohalogenation in the following halides under treatment with a strong base follows the order:

$HBr$ reacts with $CH_2=CH-OCH_3$ under anhydrous conditions at room temperature to give:

When chloroform is treated with conc. $HNO_3$,it gives:

$A$ primary alkyl halide $(a)$ with molecular formula $C_{4}H_{9}Br$ reacts with alcoholic $KOH$ to give compound $(b)$. Compound $(b)$ reacts with $HBr$ to give compound $(c)$,which is an isomer of $(a)$. When $(a)$ reacts with sodium metal,it gives compound $(d)$ with molecular formula $C_{8}H_{18}$,which is different from the compound formed when $n$-butyl bromide reacts with sodium. Identify the structural formula of $(a)$ and write the equations for all the reactions.

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

The major product $(A)$ is: