The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
The whole mass of the atom is concentrated in the nucleus. As Einstein's theory describes, energy and mass are inter-convertable. The constituents of the nucleus can't be separated for free. Conversely when these constituents bind together, they lose energy in the form of radiation which thereby decrease the mass of the bound constituents, this energy is exactly the so called binding energy of the nucleus.