Which particle has a mass of approximately 1 atomic mass unit?

A) an alpha particle

B) a beta particle

C) an electron

D) a neutron

The answer is D) a neutron.

When we say an atom's mass is, like 5 atomic mass units actually we are saying that the total number of the neutrons and protons in its nucleus is 5.

The mass of a neutron and a proton is each one atomic mass unit.

Mass of a neutron is approximately 1 atomic mass unit.