Which two types are classified under DC power sources?

The correct answer identifies DC generators and transformer-rectifiers as the two types classified under DC power sources.

A DC generator is a device that converts mechanical energy into direct current electrical energy. This conversion process is essential in various applications, from supplying power to electrical systems to charging batteries. DC generators are specifically designed to produce a steady stream of DC voltage, which is crucial for devices that require a constant power supply.

The transformer-rectifier combination plays an important role in converting alternating current (AC) into direct current (DC). The transformer steps down (or steps up) the AC voltage to a desired level, and the rectifier then converts this AC voltage into DC voltage. This process is commonly used in power supply systems where DC is needed after stepping down AC voltage.

In contrast, other options include components that do not fit the classification of DC power sources as defined in the context. For instance, a transformer is inherently an AC device designed to work with alternating current and doesn't generate or supply DC power on its own. Similarly, capacitors are energy storage devices but do not directly classify as power sources.