(a)
$CH_{3}NH_{2}\underset{- HI}{\overset{\ \ \ \ CH_{3}I\ \ \ \ \ }{\rightarrow}}\left( CH_{3} \right)_{2}NH\underset{- HI\ \ }{\overset{CH_{3}I\ \ \ \ }{\rightarrow}}\left( CH_{3} \right)_{3}N\overset{CH_{3}I\ \ }{\rightarrow}\left( CH_{3} \right)_{4}N^{+}I^{-}$
Hence, three molecules of $CH_{3}I$ is used.