Among the following, the surfactant that Will form micelles in aqueous solution at the lowest molar concentration at ambinent condition is:

The critical micelle concentration (CMC) in colloidal systems is the level of surfactant concentration above which micelles form and all further surfactants added to the system produce micelles.
Chain length has an impact on CMC. The CMC is smaller the longer the hydrocarbon tail chain is. Among all,

${\mathrm{CH}}_3{\left({\mathrm{CH}}_2\right)}_{15}{\mathrm{N}}^{+}{\left({\mathrm{CH}}_3\right)}_3{\mathrm{Br}}^{-}$ most extensive carbon chain. It therefore has the lowest CMC.