Which one of the following is massless?

A. Proton
B. Electron
C. Neutron
D. Photon
the photon is a massless particle. According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons, it was known that light carries momentum and will exert pressure on a surface. This is not evidence that it has mass since momentum can exist without mass.