The answer is D
Nuclear Fission means to split or break apart the nucleus of an atom. Nuclear Fusion means to combine the nuclie of 2 atoms. Since Uranium is such a heavy element (the heavies natural element) it can only undergo fission as it breaks up into lighter elements. Similarly Hydrogen, the lightest element, cannot be split and but can combine with other hydrogen as it undergoes fusion.
B and C are associated in both types of nuclear reactions