DC power is supplied by ________.

In the context of the CRJ550 systems, the correct answer is that DC power is supplied by four transformer rectifier units. The aircraft's electrical system utilizes transformer rectifier units (TRUs) to convert AC power to DC power, which is necessary for the operation of various DC-dependent systems on the aircraft. Each TRU is capable of providing a certain amount of DC power, ensuring redundancy and reliability in the system.

By having four transformer rectifier units, the aircraft design guarantees that there are multiple sources of DC power, which enhances system reliability. If one unit fails, the remaining units can compensate, maintaining adequate power supply for critical systems. This redundancy is crucial in aviation, where reliability and safety are paramount. The other options, indicating a different quantity of transformer rectifier units, would not provide the same level of redundancy or could risk the failure of DC power supply to essential systems onboard.