The correct choice is
D. electron
mass of proton is 1.67 x 10⁻²⁷ kg and has positive charge on it.
mass of neutron is slightly greater than the mass of proton. the mass of neutron is 1.675 x 10⁻²⁷ kg and has no charge on it.
mass of electron is 9.31 x 10⁻³¹ kg
comparing the masses of electron, proton, neutron and the nucleus, we see that the mass of electron is smallest.