In the given circuit, the equivalent resistance between A and B (if A has positive potential and B has negative potential) is

→Current passes from A to C .→So +Ve  polarity reaches to N side of diode→So diode is in Reverse biasin between B & D there is no resistorin between A & C there is no resistorDiode is in Reverse biasR1,R2  are in parallel R12=R2  R12,R4  are in parallelR124=R12×R4R12+R4=R2×RR2+R =R22R+2R2=R23R=R3Ω