Which alkyl halide undergoes the ${S}_{N}1$ reaction most rapidly?

The order of reactivity of the following alkyl bromides towards $S_N1$ reactions is:
$a$) $(CH_3)_2CHBr$
$b$) $CH_3CH_2Br$
$c$) $(CH_3)_3CBr$

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:

In which of the following reactions is the $Saytzeff$ alkene the major product?

When $HNO_3$ is reacted with $ . . . . . . $,chloropicrin is formed.

$CH_3-C(CH_3)=CH-CH_2-CH_2-Cl \xrightarrow[CaCO_3]{H_2O} (X)$
The major product $(X)$ of the above reaction is: