Correct Answer - D
When positive of external battery is connected to A and negative of battery to B then `D_1` is forward biased, will offer least resistance. `D_2` is reverse biased will offer maximum resistance. The effective circuit will be as shown in figure.
The effective resistance is
`=(2xx9)/(2+9)+4=5.64 Omega`
When negative of external battery is connected to A an positive of battery to B, then `D_1` is reverse biased and `D_2` is forward biased. The effective circuit will be as shown in figure
The effective resistance is
`=4+(9xx8)/(9+8)=8.24 Omega`