Refer to figure.
Nowadays, commercial aircraft electrical system is supplied by two (or more) engine-driven generators regulated in speed by integrated drives. These generators’ characteristics are:

• 115/200 Volts, 3-phase AC.
• 400 Hz.
• 90 kVA.

28 V DC is obtained by converting the generators’ output AC via the transformer rectifier units. The APU generator is essentially the same as the main engine-driven generators, so it will provide a very similar output (sometimes with a lower current output due to the size and power of the APU). The APU will electrically supply the aircraft and other associated systems on the ground when the engines are not running, and the Ground Power unit (GPU) is not connected.

As for the options:

• "is driven at constant speed through a constant speed drive (CSD), in the same way as the main AC generator."
  • INCORRECT --> APUs usually run at a constant RPM, so do not require a CSD, just a standard gearbox, for the AC generator produce the correct 115V, 400 Hz output.
• "must have the same characteristics as the main AC generator so that it can be easily coupled with the latter."
  • INCORRECT --> It is quite rare to need to "couple" (parallel) generators together, and even moreso for the APU generator as it is usually smaller that the main AC generators for the aircraft.
• "is excited by its Generator Control Unit (GCU) as soon as the APU starts up.
  • INCORRECT --> The Generator Control Unit simply controls the generator, it does not provide the excitation current for it (but may limit the excitation current). The generator is also not excited until the APU is fully stable.
• "supplies the aircraft with three-phase 115 V, 400 Hz AC."
  • CORRECT --> As mentioned above, the APU AC generator provides the same output as the main engine generators, three-phase 115 V, 400 Hz AC.