Which among the following is most common in alkyl halides?

Alkyl halide can be converted into alkene by

The relative reactivity of the following halides toward $S_{N}2$ reaction follows the order:
$(P)$ $CH_2=CH-Cl$
$(Q)$ $CH_3-CH_2-Cl$
$(R)$ $CH_3-O-CH_2-CH_2-Cl$
$(S)$ $C_6H_5-CH_2-Cl$

Which among the following compounds will undergo the fastest $S_{N}2$ reaction?

$tert$-Butylbromide reacts with $aq.$ $NaOH$ by $S_N1$ mechanism while $n$-butylbromide reacts by $S_N2$ mechanism. Why?

Compound $(A), \, C_8H_9Br,$ gives a white precipitate when warmed with alcoholic $AgNO_3.$ Oxidation of $(A)$ gives an acid $(B), \, C_8H_6O_4.$ $(B)$ easily forms anhydride on heating. Identify the compound $(A).$