Which of the following is not an example of a subatomic particle? electron photon neutron proton

The correct answer is a photon.  

A photon refers to a particle signifying a quantum of light or other electromagnetic radiation. A photon carries energy equivalent to the radiation frequency, however, exhibits zero rest mass.  

A subatomic particle refers to a unit of energy or matter, that is, the basic makeup of all the matter. On the basis of modern atomic theory, an atom exhibits a nucleus, also known as the core or its center. The nucleus comprises subatomic particles, that is, neutrons and protons. The neutrons are neutral particles, and the protons are positively-charged particles. A cloud of very small subatomic particles known as electrons surrounds the nucleus. The electrons are negatively charged particles.  

it is b that is the answer.