Heavier nuclei having large mass number has large nuclei radius. Attractive nuclear force, which has small range, is not effective in heavier nuclei due to the large nuclei radius. Repulsive Coulomb forces between protons will be now effective since these forces have longer range. Repulsive force causes instability of nuclei.

Stability is achieved by having more neutrons than protons in the nuclei. More neutrons (not exerting electric repulsion) in the nuclei or larger N/Z ratio do make the range between the nucleon-pairs comparable to the nuclear forces. Hence, attractive nuclear forces dominate over repulsive coulomb forces and thus stability is achieved. Therefore, Option C and D both are correct.