Answer: 430 nm
Explanation:
For 257 kJ to dislodge one mole of electrons we need,
275 x 10^3 / 6 x 10^23 = 4.6 x 10^-19
Using Einstein’s relationship between energy, frequency and wavelength
E = hf = h x (c/landa)
Therefore
Landa = h x (c/E) = 6.6 x 10^-34 x (3 x 10^8 / 4.6 x 10^-19 = 4.3 x 10-7 m.
In nm, landa = 430 nm