Which of the following is a key characteristic of air brakes?

Air brakes are designed to function through the principles of air pressure, making option B the correct choice. In air brake systems, compressed air is utilized to apply the brakes. When the driver activates the braking system, air is sent through the brake lines to engage the brakes on the train's wheels. This method allows for efficient and powerful braking capabilities, as the system can apply significant force quickly across all the brakes.

The reliance on air pressure is what distinguishes air brakes from other types of braking systems, such as hydraulic brakes or mechanical lever systems. This pressure-based operation allows for effective braking over long distances and is particularly suited for trains, which require a system capable of managing substantial weight and speed.

In contrast, the other options pertain to features that do not align with the operational characteristics of air brakes. Mechanical levers are typically associated with older or alternative braking systems, while hydraulic brakes utilize fluid dynamics rather than air pressure. Additionally, modern air brakes are often automatically controlled through advanced systems, though manual control may be employed for more nuanced operations. However, the key defining element of air brakes remains their reliance on air pressure for effective operation.