Answer : $\;CH_{3}(CH_{2})_{15}N^{+}CH_{3}Br^{-}$
Explanation :
Greater is the size of the hydrophobic chain , less is the solubility in water and greater is the frequency of surfactant molecules to associate to form micelle . Hence $\;CH_{3} (CH_{2})_{15} N^{+}CH_{3}Br^{-}\;$ with the longest carbon chain has the lowest C .M .C