Which of the following compounds undergoes nucleophilic substitution reaction most easily via the $S_{N}1$ mechanism?

$1$-methyl ethylene oxide (propylene oxide) when treated with an excess of $HBr$ produces:

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

Explain the stereochemistry of $S_{N}2$ reaction with a suitable example.

Arrange the following halides in decreasing order of $S_N1$ reactivity:
$(I)$ $CH_3CH_2CH_2Cl$
$(II)$ $CH_2=CHCHClCH_3$
$(III)$ $CH_3CH_2CHClCH_3$

Prolonged exposure of chloroform in humans may cause damage to the liver. It is due to the formation of the following compound.