Correct Answer: neutron

Solution : The correct option is neutron.

The neutron has the greatest mass among the proton, neutron, electron, and nucleus of hydrogen. The mass of a proton is 1.672621777(74) × 10 ^-27 kg. The mass of a neutron is 1.674927498(94) × 10 ^-27 kg. The mass of an electron is 9.1093837015(28) × 10 ^-31 kg. The mass of a hydrogen nucleus is 1.674927498(94) × 10 ^-27 kg. A neutron's mass is somewhat larger than that of a proton. An electron's mass is substantially lower than that of a proton or neutron. Because a hydrogen nucleus contains just one proton, its mass is the same as that of a proton.