Which of the following species is most reactive in $S_{N}2$ reaction?

Chloroform is slowly oxidized by air in the presence of light to form:

Consider the reaction shown below. The number of $\pi$ electrons present in the major product '$P$' is ...... .

Consider all the structural isomers with molecular formula $C_5H_{11}Br$. They are separately treated with $KOH$ $(aq)$ to give respective substitution products, without any rearrangement. The number of products which can exhibit optical isomerism from these is . . . . . . .

The organic chloro compound,which shows complete stereochemical inversion during an $S_{N}2$ reaction,is

Identify $(B)$ and $(C)$ and how are $(A)$ and $(C)$ related?