What do automotive relays primarily control?

Automotive relays primarily control high-powered devices like headlights and cooling fans due to their ability to handle higher currents while being activated by lower-power signals. A relay functions as an electrically operated switch that allows a small current from a control circuit to manage a larger current circuit, enabling the operation of components that require more electrical power than the controls (like switches from the driver's cabin) can provide. This design protects sensitive components and wiring from overheating and potential damage that could result from high currents.

In contrast, the other options represent functionalities that either do not rely on relays or do not encompass their primary purpose. Low-powered devices, such as indicator lights or small sensors, typically do not require the use of relays since they can be controlled directly by the low-power circuits. While relays might be involved in systems that interact with the vehicle's braking system or battery charging processes, these are not the primary functions of automotive relays. The main role of relays is indeed to facilitate the operation of high-powered devices, making option B the most accurate answer.