The current flows from positive to negative, meaning it flows from the bigger line on the battery to the smaller line on the other side of the battery. It flows clockwise.
The diode breaks the circuit. If the switch on the branch is open, then it means that no current is flowing to the circuit. So the ammeter would show 0.
If the switch is closed, it means that current flows through the branch but not on the side in which the diode is placed. Current would equal to voltage divided by resistance. We know resistance equals 3.
Current = 6/3 = 2A
So B is correct.