If the speed of photoelectrons is $10^4\: m/s$, what should be the frequency of the incident radiation on a potassium metal? (work function of potassium = $2.3 eV)$

Question

$\begin {array} {1 1} (a)\;5.6 \times 10^{15}Hz & \quad (b)\;0.56 \times 10^{15}Hz \\ (c)\;0.56 \times 10^{17}Hz & \quad (d)\;5.6 \times 10^{17}Hz \end {array}$