The mass defect (ΔM) for a nucleus of mass number (A), atomic number (Z) and mass (M) is given by

$AM = [Z m_p + (A - Z) m_n] - M$

The correct answer is Option (4) → $AM = [Z m_p + (A - Z) m_n] - M$

Mass defect is given by:

$\Delta M = [Z m_p + (A - Z) m_n - M]$

where,

$m_p$ = mass of proton

$m_n$ = mass of neutron

$M$ = actual mass of nucleus

$A$ = mass number

$Z$ = atomic number