Which is most reactive for $S_{N}2$ reaction?

The formation of butane nitrile is possible in which of the following reactions?

Increasing the concentration of an electrophile (substrate) in a typical $S_N2$ reaction by a factor of $3$ and the concentration of the nucleophile by a factor of $3$ will change the reaction rate to

What is the product of the reaction given below?

Which of the following compounds will not give a precipitate with $AgNO_3$?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$ :
Assertion $(A)$ : $S_N2$ reaction of $C_6H_5CH_2Br$ occurs more readily than the $S_N2$ reaction of $CH_3CH_2Br$.
Reason $(R)$ : The partially bonded unhybridized $p$-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring.
In the light of the above statements,choose the most appropriate answer from the options given below: