Spin quantum number for the last electron in potassium is?

$+ \frac{1}{2}$
Electronic configuration of Potassium ($\text{_19"K}$) is
${\text{1s"^2\ "2s"^2\ "2p"^6\ "3s"^2\ "3p"^6\ "4s}}^{1}$
Last electron is in $\text{4s}$ sub-shell. As a convention spin quantum number of first entering electron in an orbital is assigned as $+ \frac{1}{2}$