An alternator is an AC generator in which a rotating magnetic field (produced by the rotor or field winding) induces alternating current in the fixed stator windings. By design, the output of the stator windings is always alternating current (AC) because the rotating magnetic flux reverses polarity with each half revolution of the rotor. 

However, most aircraft electrical systems, particularly on light general aviation aircraft, operate on direct current (DC) at 28 volts. The aircraft battery is a DC device and most aircraft electrical loads are designed for DC supply. To make the alternator output compatible with the aircraft DC electrical system, the AC output from the stator must be converted to DC. This conversion is performed by a rectifier, which is an assembly of diodes that allow current to flow in one direction only. In a three-phase alternator the rectifier typically consists of six diodes arranged in a full-wave bridge configuration, converting three-phase AC into a smooth DC output.

Vary the field current in order to control the generator output → INCORRECT. This is the function of the VOLTAGE REGULATOR, not the rectifier. The voltage regulator senses the alternator output voltage and adjusts the current supplied to the field winding to maintain a constant output voltage regardless of engine speed or electrical load. 

Control the voltage output → INCORRECT. Voltage regulation is performed by the voltage regulator, a separate electronic control unit that adjusts field excitation current. The rectifier does not regulate or control voltage. It is a passive diode assembly that performs a signal conversion function only, changing the waveform from AC to DC.

Energise the rotor with the battery power → INCORRECT. Initial rotor excitation from the battery may be required in some alternator designs at startup (before the alternator is self-sustaining), and this function is managed through the field circuit and sometimes a separate excitation circuit. This has nothing to do with the rectifier.

Convert the AC output into DC → CORRECT. The rectifier is a diode assembly positioned at the output of the alternator stator windings. Its sole function is to convert the alternating current produced by the stator into the direct current required by the aircraft electrical system and battery.