Which statement about subatomic particles is true? protons are the only subatomic particles to have charge. electrons are the subatomic particles with the smallest mass. neutrons orbit the nucleus of the atom. subatomic particles all have the same mass

The correct option is this: ELECTRONS ARE THE SUB ATOMIC PARTICLES WITH THE SMALLEST MASS.
The atom is made up of three sub particels, which are protons, neutrons and electrons. Both proton and neutrons have the atomic mass of 1 unit while the mass of the electron is 0.0005.

The correct answer is B) Electrons are the subatomic particles with the smallest mass. 

Neutrons and protons weigh roughly the same, and comprise the vast majority of the atomic mass of an atom. The electron is several orders of magnitude less massive than the neutron and proton, and has almost not effect on the mass of the atom. 

Why the other answers are wrong:
Subatomic particles all have the same mass - see paragraph above
Neutrons orbit the nucleus of the atom: Neutrons are found in the nucleus, along with protons, but they do not orbit it.
Protons are the only particles with charge. Electrons have a charge as well.