Which of the following reactions is an example of a nucleophilic substitution reaction?

The reactivity of ethyl chloride towards nucleophilic substitution is:

For the given reaction,what is the major product '$A$'?
$3-\text{ethylbenzonitrile} + Br_2 \xrightarrow{UV \text{ light}} A$

Which of the following compounds gives the same $S_N1$ and $S_N2$ product?

Conversion of $I$ to $II$:

$A$ 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.