For a photon (massless particle), the energy-momentum relation simplifies to [latex]E = PC[/latex], so [latex]E^2 = P^2C^2[/latex] — option (b).

The cut-off voltage relates to frequency as [latex]eV_o = hf - \phi_o[/latex], i.e., [latex]V_o = \dfrac{h}{e}f - \dfrac{\phi_o}{e}[/latex]. This is a straight line with a positive slope [latex]\dfrac{h}{e}[/latex] that does not pass through the origin — it has a positive intercept on the f-axis (the threshold frequency) and a negative intercept on the V₀-axis. This matches graph (c), which shows a straight line offset from the origin with a positive f-intercept.