What do you mean by dehydrohalogenation?

The rate of $\beta$-dehydrohalogenation in the following halides under treatment with a strong base follows the order:

For the following compounds,the correct statement$(s)$ with respect to nucleophilic substitution reactions is(are):
$I$: Benzyl bromide $(C_6H_5CH_2Br)$
$II$: Cyclohexylmethyl bromide $(C_6H_{11}CH_2Br)$
$III$: tert-Butyl bromide $((CH_3)_3CBr)$
$IV$: $1-$Phenylethyl bromide $(C_6H_5CH(CH_3)Br)$
$A$. $I$ and $III$ follow $S_{N}1$ mechanism
$B$. $I$ and $II$ follow $S_{N}2$ mechanism
$C$. Compound $IV$ undergoes inversion of configuration
$D$. The order of reactivity for $I$,$III$ and $IV$ is: $IV > I > III$

Differentiate between the $S_N2$ and $S_N1$ mechanisms of alkyl halides.

Which is correct for the above reaction $:-$

The product $(C)$ in the following reaction sequence is:
$4-\text{Bromoethylbenzene}$ $\xrightarrow[\Delta]{Br_2} B$ $\xrightarrow{\text{aq. } KOH} C$