Which of the following technologies is commonly used in high-voltage transmission systems?

The correct choice highlights the use of AC substations, which play a crucial role in high-voltage transmission systems. AC substations are essential for transforming and distributing electrical energy at high voltages. They serve multiple functions, including voltage regulation, power factor correction, and switching operations, which are vital for maintaining the stability and efficiency of the grid.

High-voltage transmission systems often utilize alternating current (AC) due to its advantages in reducing energy losses over long distances and its ability to easily transform voltage levels. AC substations typically include equipment such as transformers, circuit breakers, and protective relays that facilitate the control and distribution of electrical power.

The other options do not align with the primary functions and technology of high-voltage transmission systems. For instance, while DC motors are used in specific applications, they are not a standard component of high-voltage transmission. Step-down transformers, although important in transforming high voltage to lower levels for distribution, do not specifically indicate the nature of high-voltage transmission systems as well as AC substations do. Additionally, low-voltage conductors are not relevant to the high-voltage context, as they are primarily used in residential and commercial applications rather than in the transmission of electricity over long distances.