Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
Mass number is always equal to the number of protons plus the number of neutrons and Atomic number is always equal to the number of protons which is equal to the number of electrons to maintain the neutrality of the atom.
Hence, option D is correct.