Which among the following compounds will undergo the fastest $S_{N}2$ reaction?

The order of relative reactivity of the given halides towards $S_N2$ reaction is :

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:

Which of the following nucleophiles will show minimum reactivity towards $S_N2$ reaction:

Tertiary alkyl halide is practically $inert$ to substitution by $S_{N}2$ mechanism because of:

The reactivity of ethyl chloride towards nucleophilic substitution is: