Under the specified conditions,substrate $X$ undergoes substitution and elimination reactions to give products $A - D$. $A$ and $B$ are stereoisomers,but not enantiomers. $C$ and $D$ are enantiomers. $A$ is not an isomer of $C$. Which of the following could be the starting material $X$ ?
$X \xrightarrow[\Delta]{H_2O} A + B + C + D$

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$ :
Assertion $(A)$ : $S_N2$ reaction of $C_6H_5CH_2Br$ occurs more readily than the $S_N2$ reaction of $CH_3CH_2Br$.
Reason $(R)$ : The partially bonded unhybridized $p$-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring.
In the light of the above statements,choose the most appropriate answer from the options given below:

Among the following,the compound which undergoes the fastest solvolysis is

Arrange the following organic halides in the correct order of reactivity towards $S_{N}2$ displacement:
$(P) \ (CH_3)_2C(Br)CH_2CH_3$
$(Q) \ BrCH_2(CH_2)_3CH_3$
$(R) \ CH_3CH(Br)(CH_2)_2CH_3$

Explain the mechanism of a unimolecular nucleophilic substitution $(S_N1)$ reaction with a suitable example.

Two isomers $(A)$ and $(B)$ with molar mass $184 \ g/mol$ and elemental composition $C, 52.2 \ \%; H, 4.9 \ \%$ and $Br, 42.9 \ \%$ give benzoic acid and $p$-bromobenzoic acid,respectively,on oxidation with $KMnO_4$. Isomer '$A$' is optically active and gives a pale yellow precipitate when warmed with alcoholic $AgNO_3$. Isomer '$A$' and '$B$' are,respectively .... .