Which one of the following is a common feature of both transformers and transistors?

The correct answer highlights that both transformers and transistors have a role in regulating electrical signals. Transformers are used to change the voltage levels of alternating current (AC) while maintaining the power transfer, allowing for the regulation of voltage and current in electrical circuits. They can shape waveforms and control signal levels, which can provide a form of regulation.

Transistors also regulate electrical signals but through different means. They can amplify signals by controlling the flow of current and voltage levels in a circuit, making them fundamental components in signal processing and gain applications.

While the other options mention functions such as amplification, increasing voltage, and converting AC to DC, these aspects do not apply universally to both devices. Amplification is characteristic of transistors but not of transformers, which mainly focus on voltage and current regulation rather than amplification. Similarly, while transformers do increase voltage, they do not amplify signals in the way transistors can, and they do not convert AC to DC—this function is specific to rectifiers. Therefore, the regulation of electrical signals emerges as the common feature that accurately encapsulates the role that both transformers and transistors play in electronic circuits.