Answer: The given statement is true.
Step-by-step explanation:
Mass defect is defined as the difference in the mass of an atom and the sum of number of protons, neutrons and electrons present in the atom.
Binding energy present inside the nucleus is responsible for holding the neutrons and protons together in an atom.
But when nuclear binding energy is not strong enough then the atom becomes unstable and loses some of its mass to produce energy.
For example,
Thus we can conclude that the statement, in a nuclear reaction mass must be lost to produce energy, is true.