$13.$ For a liquid$-$liquid extraction process, if the distribution coefficient $\backslash ($ K$\_$d $\backslash )$ is $4$ and the concentration of solute in the aqueous phase is $0\mathrm{.}25$ M$,$ the concentration in the organic phase is $\_\_\_\_\_$ M$.$

$-$ a$)0\mathrm{.}1$

$-$ b$)0\mathrm{.}5$

$-$ c$)1$

$-$ d$)4$