Which of the following is most reactive toward $S_N2$ reaction?

Reaction $(A)$:
$Ph-CH=CH-CH_3 \xrightarrow[H_2O_2 / OH^-]{B_2H_6 / THF}$ products.
Reaction $(B)$:
$CH_3-CH=CH-Ph \xrightarrow[NaBH_4]{Hg(OAc)_2 / H_2O}$ products.
Correct statement is:

For the following reactions:
$CH_{3}CH_{2}CH_{2}Br + Z^{-} \xrightarrow[substitution]{k_s} CH_{3}CH_{2}CH_{2}Z + Br^{-}$
$CH_{3}CH_{2}CH_{2}Br + Z^{-} \xrightarrow[elimination]{k_e} CH_{3}CH=CH_{2} + HZ + Br^{-}$
where
$Z^{-} = CH_{3}CH_{2}O^{-} (A)$ or $(CH_{3})_{3}CO^{-} (B)$
$k_s$ and $k_e$ are,respectively,the rate constants for the substitution and elimination,and $\mu = \frac{k_s}{k_e}$,the correct option is:

Consider the given reaction: $CH_3-CH=CH-CH_2-OH \xrightarrow[S_N1]{HBr} [P]$. In the given reaction,the product $[P]$ is:

Among the following organic halides,arrange them in the increasing order of their dehydrohalogenation reactions in the presence of alcoholic $KOH$:
$(A)$ $CH_3-CH_2-Br$
$(B)$ $CH_3-CH_2-CH_2-Br$
$(C)$ $CH_3-CH(Br)-CH_3$
$(D)$ $(CH_3)_3-CBr$

Which of the following represents the correct order of density?