The correct option is A K4[Fe(CN)6]
Ionic conductance increases with increasing the number of ions, produced after decomposition.
CompoundNo. of ions producedK4[Fe(CN)6]5[Co(NH3)6]Cl34[Cu(NH3)4]Cl23[Ni(CO)4]0
So here the compound with maximum ionic conductivity is K4[Fe(CN)6]