The energy of a photon is given by,

$\mathrm{E}=\mathrm{hv}=\frac{\mathrm{hc}}{\mathrm{\lambda }}$ ...(i)

de-Broglie wavelength is given by,

$\mathrm{\lambda }=\frac{\mathrm{h}}{\mathrm{p}}\Rightarrow \mathrm{p}=\frac{\mathrm{h}}{\mathrm{\lambda }}$

Putting this in (i) we get, the energy of a photon as E = pc

The equation E = pc is valid only for a particle with zero rest mass. The rest mass of a photon is zero, but the rest mass of an electron is 9.1 × 10^–31 kg.

Hence, the equation E = pc is valid for a photon but not for an electron.

Hence, (C) is the correct answer.