The correct option is B [Cu(NH3)4]SO4
Coordination complex is classified into positive, negative and neutral complex based on the charge on coordination sphere.
K2[Cd(CN)4] dissociate as [Cd(CN)4]2− and 2K+. The charge on coordination sphere is -2. Hence, it is a anionic complex.
[Cu(NH3)4]SO4 dissociate as [Cu(NH3)4]2+ and SO2−4. The charge on coordination sphere is +2. Hence, it is a cationic complex.
K3[Fe(CN)6] dissociate as [Fe(CN)6]3− and 3K+. The charge on coordination sphere is -3. Hence, it is a anionic complex.
Charge on coordination sphere of [Ni(CO)4] is absent. Hence, it is neutral complex.
Hence, (b) is correct.