A stock had returns of $9\%,3\%,4\%,15\%$ over the past $4$ years. What is the standard deviation of this stock for the

past four years?

$5\mathrm{.}4\%$

$5\mathrm{.}9\%$

$6\mathrm{.}3\%$

$6\mathrm{.}6\%$

$7\mathrm{.}6\%$