Among $CH_3F$,$CH_3Cl$,$CH_3Br$,and $CH_3I$,which one does not give methane on reduction?

Consider the following bromides.
The correct order of $S_N1$ reactivity is

The product $(C)$ in the following reaction sequence is:

Arrange the leaving group ability of the following groups in decreasing order:
$(I)$ $CH_3-C_6H_4-SO_3^-$,$(II)$ $C_6H_5-SO_3^-$,$(III)$ $N_3^-$,$(IV)$ $Br^-$

Among the given halides,which one will give the same product in both $S_{N^1}$ and $S_{N^2}$ reactions?
$(I) \ CH_3-CH(CH_3)-CH_2-CH(Br)-CH_3$
$(II) \ \text{2-methylcyclohexyl chloride}$
$(III) \ \text{Chlorocyclohexane}$
$(IV) \ CH_3-CH(Br)-CH_2-CH_3$

Which of the following is most reactive towards nucleophilic substitution reaction?