I3 is zero as the diode in that branch is reverse bised. Resistance in the branch AB and EF are each (125 + 25)Ω = 150Ω.
As AB and EF are identical parallel branches, their effective resistance is 150/2=75Ω
Net resistance in the circuit = (75 + 25) Ω = 100Ω .
As resistances of AB and EF are equal, and I1 = I2 + I3 + I4, I3 = 0