Which of the following haloalkanes is most reactive towards nucleophilic substitution reactions?

Identify $A$ and $B$ in the following sequence of reaction:

Given below are two statements $:$
Statement $(I) :$ Alcohols are formed when alkyl chlorides are treated with aqueous potassium hydroxide by elimination reaction.
Statement $(II) :$ In alcoholic potassium hydroxide,alkyl chlorides form alkenes by abstracting the hydrogen from the $\beta-$carbon.
In the light of the above statements,choose the most appropriate answer from the options given below $:$

Which of the following,when undergoing addition of $HBr$,will form $ONLY$ a pair of diastereomers?

$A$ mixture of $1-$chloropropane and $2-$chloropropane when treated with alcoholic $KOH$ gives:

The reactivity of ethyl chloride towards nucleophilic substitution is: