What do you mean by dehydrohalogenation?

Which of the following,on heating with aqueous $KOH$,produces acetaldehyde?

$MMPP \to$ Magnesium monoperoxyphthalate. Product $(X)$ is

$A$ mixture of an alkyl halide and an aryl halide is treated with sodium in dry ether. What is the name of this reaction?

The major product $(A)$ of the following reaction is:

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: $IV > I > III$