In the Theory of Special Relativity, the momentum of a massless particle is equal to which of the following?
A. m0c^2
B. E2c^2
C. E/c
D. E/c^2
Just from units:
m c^2 is energy, mass times velocity squared
momentum is mass times velocity, which is energy / velocity
m c^2/c = m c = E/c = p
note equation 1. on the opening page of this link
https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation