Which statement correctly describes the isomerism associated with the given complex ions?
a) [Ni(H₂O)₅NH₃]₂ exhibits geometric isomerism.
b) [Ni(H₂O)₄(NH₃)₂]₂ displays cis-trans isomerism.
c) [Ni(H₂O)₃(NH₃)₃]₂ shows optical isomerism.
d) All of the above exhibit isomerism.