Dehydrohalogenation of an alkyl halide is a/an

Which among the following alkyl halides gives more than one product upon dehydrohalogenation?

$HBr$ reacts with $CH_2=CH-OCH_3$ under anhydrous conditions at room temperature to give:

The rate of $S_{N}1$ reaction for the given compounds is:

The order of reactivity of the following compounds towards the $S_{N}1$ reaction will be:
$(I)$ Benzyl chloride
$(II)$ $p$-Methoxybenzyl chloride
$(III)$ $p$-Nitrobenzyl chloride

What is retention and inversion of configuration? Explain with suitable examples.