The stopping potential $\left(V_0\right)$ versus frequency plot of a substance is shown in figure. The threshold wavelength is

 

From the given plot, threshold frequency,  ${\nu }_0=5\times {10}^{14} \mathrm{Hz}$

Since, $c={\nu }_0\lambda$, threshold wavelength  ${\lambda }_0=\frac{c}{{\nu }_0}=\frac{3\times {10}^8}{5\times {10}^{14}} \mathrm{m} = 0.6\times {10}^{-6} \mathrm{m} = 6000 Å$