Transformer Rectifier Units (TRU’s) consists of a unit that combines a transformer and a rectifier. Therefore it converts Alternating Current (AC) into Direct Current (DC) whilst at the same time transforming a certain voltage into another. These units are used in situations where AC distribution system needs to supply a DC system.

The other answers are incorrect:

• “Rectifiers and voltage reducing resistors” – Even though rectifiers are commonly used to convert AC to DC, using resistors to lower the voltage have many disadvantages. Firstly, resistors dissipate power as heat, therefore the system will lose energy to lower the voltage, this is something that transformers typically do not. Secondly, the voltage drop of a resistor is dependent on the load, therefore cause fluctuations in voltage. This is something that doesn’t typically happen with transformers.
• “AC-DC Regulators” – Regulators work very similarly to resistors, hence they lose energy by turning it into heat. Therefore they are much less efficient than transformers.
• “Portable batteries” – Even though portable batteries would be able to power the DC power, and it is required that batteries can power the main system in case of an emergency, they would not be able to power the aircraft for the entire flight. Therefore DC batteries would never entirely power the DC system unless in an emergency.