Final answer:
The rotor in an alternator is the rotating electromagnet that provides the magnetic field necessary to induce voltage and current in the stator.
Step-by-step explanation:
In an alternator, the component that is a rotating electromagnet, providing the magnetic field to induce voltage and current in the stator, is the rotor. Alternators function by rotating a coil within a magnetic field, which is similar in principle to the workings of an electric motor. The rotation of the rotor within the stator's windings generates an alternating current (AC) due to electromagnetic induction. The other options given, such as the rectifier bridge, voltage regulator, and pulley drive, perform different functions within the alternator. The rectifier bridge converts AC to direct current (DC), the voltage regulator controls the alternator's output voltage, and the pulley drive connects the alternator to the engine's rotational power.