The mass defect is the result of what action occurring?

The mass defect is the result of what action occurring?

A. energy being converted into moles of atoms when neutrons break apart protons
B. mass being converted to energy when protons and neutrons bind together in a nucleus
C. mass being converted to energy when protons and neutrons break apart in a nucleus
D. protons and electrons being attracted to each other
E. strong forces overcoming weak forces