The nucleus of an atom contains positively charged particles, called protons, and neutral particles, called neutrons. how is the nucleus of an atom held together? a. electric forces exist between the protons and neutrons in a nucleus, and the particles are attracted to one another by these electric forces. b. electric forces exist between the protons of the nucleus, and the protons are attracted to one another by these electric forces. c. a strong force exists between the particles of the nucleus, but this force is always much weaker than the electric forces that would pull them apart. d. a strong force exists between the particles in a nucleus, and this force is usually stronger than the electric forces that would pull them apart.