Answer
The correct answer is:
(c) $\frac{hc}{\phi}$
Work Step by Step
If the work function is $\phi$, we can find an expression for the threshold wavelength:
$hf = \phi$
$\frac{hc}{\lambda} = \phi$
$\lambda = \frac{hc}{\phi}$
The correct answer is:
(c) $\frac{hc}{\phi}$
