In Fig assuming the diodes to be ideal:
| A. |
`D_1` is forward biased and `D_2` is reverse biased and hence current flows from A to B |
| B. |
`D_2` is forward biased and `D_1` is reverse biased and hence no current flows from B to A and vice versa |
| C. |
`D_1` and `D_2` are both forward biased and hence current flows from A to B |
 |
| D. |
`D_1` and `D_2` are both reverse biased and hence no current flows from A to B and vice- versa |