The answer is option (b)
It is positively charged and includes most of the atom's mass.
Atoms are the basic units of matter and they are composed of a nucleus
and one or more electrons bound to the nucleus.The atomic nucleus is the small,
dense region at the center of an atom which consists of one or more protons and a similar number of neutrons.The protons are positively electric charged, the electrons have a negative electric charge, and the neutrons have no electric charge. Protons and neutrons are heavier than electrons and reside in the nucleus and they are bound together to form a nucleus by the nuclear force.
More than 99.94% of an atom's mass is in the nucleus.