Verified Answer :

The mass of a proton is approximately equal to the mass of a neutron.

atoms are made of three main subatomic particles that are electrons, neutrons and protons. electrons are negatively charged and protons are positively charged and neutrons are neutral , in other words no charge. 
Protons and neutrons are located in the nucleus of the atom while electrons are  in energy shells that orbit around the nucleus
The mass of a proton is 1 atomic mass unit and mass of neutron is 1 atomic mass unit and electron is 1/1836th mass of an atomic mass unit 
protons and neutrons have approximately similar masses and both their masses together give the mass of the atom as a whole.

answer is 3) a neutron

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