The annual returns on investment were $5\%,12\%,-8\%,9\%$ and $-15\%.$ What is the standard deviation of the returns?

a$.10\mathrm{.}37\%$

b$.12\%$

c$.11\mathrm{.}59\%$

d$.3\mathrm{.}83\%$

e$.3\mathrm{.}43\%$

f$.$ None of the above