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We know that mass number of an atom is the sum of protons and neutrons inside the atom. Usually this is expressed in unified atomic mass units or 1/12 of the mass of a single C-12 (carbon-12) atom at rest.
Isotope means an atom which has same number of electrons but different neutrons.
atomic mass number `= N_p+N_n`
`2z+4 = z+N_n`
`N_n = 2z+4-z = z+4`
So there will be (z+4) neutrons in the isotope.
So the correct answer is A)
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