When the small magnetic needle is stationary, the N pole rotates to the left, that is, the magnetic induction line rotates to the left, which can be obtained from the magnetic field distribution of the energized solenoid. The right side of the electromagnet is the S pole and the left side is the N pole. According to the ampere rule, the current flows into the electromagnet from the A terminal, that is, A is the positive pole of the power supply.
So the answer is: current (or live wire); a.