Which of the following is most reactive toward $S_{N}2$ reaction?

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:

Formation of $2-$butene from $2-$bromobutane is according to

In which of the following cases is the yield via $S_{N}2$ mechanism maximum?

The following reaction is described as:
$CH_3(CH_2)_5-CH(CH_3)-Br + OH^- \rightarrow HO-CH(CH_3)(CH_2)_5CH_3 + Br^-$

Among the following chloro-compounds,the one having the lowest dipole moment is: