When a metallic surface is illuminated with monochromatic light of wavelength $\;\lambda\;$ and stopping potential for photo electron current is $\;3V_{0}\;$ . When some metallic surface is illuminated with a light of wavelength $\;2 \lambda\;$ , The stopping potentials is $\;V_{0}\;$ . The threshold wavelength for surface is

Question

$(a)\;6 \lambda\qquad(b)\;4 \lambda\qquad(c)\;\large\frac{4 \lambda}{3}\qquad(d)\;8 \lambda$