The correct answer is option A: Subtract the atomic number from the mass number.
An atom is composed of a proton, a neutron and an electron. These are collectively called subatomic particles.
The proton and neutron are found in the nucleus of the atom, surrounded by electrons.
The atomic number is the same as the number of protons in the atom or the number of electrons in the neutral atom.
The mass number is the sum of the number of protons and the number of neutrons in the atom.
Hence, to obtain the number of neutron, we subtract the atomic number from the mass number.
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