A) proton
This is simply a matter of actually learning the terms. To wit
A) proton - One of two different particles found in the nucleus of an atom. They have about the same mass as a neutron, and have a positive charge.
B) photon - The basic unit of light.
C) neutron - The other particle found in the nucleus of an atom. They have no electrical charge and are about the same mass as a proton.
D) electron - The particle that "orbits" the nucleus of an atom. They're much lighter than either protons or neutrons, and have a negative charge.
