A very small region situated in the centre of atom, is called a/an
nucleus
Explanation for correct option:
(b) The small region situated in the center of an atom is called a nucleus.
Explanation for incorrect option:
(a) The protons and neutrons collectively present inside the nucleus, are called nucleons.
(c) The nucleus of an atom is also called as the central core of the atom.
Hence, option (b) is correct.